diff --git a/app/controllers/match_scores_controller.rb b/app/controllers/match_scores_controller.rb
index 502da1f..7d3af67 100644
--- a/app/controllers/match_scores_controller.rb
+++ b/app/controllers/match_scores_controller.rb
@@ -13,14 +13,38 @@ class MatchScoresController < ApplicationController
 
   # PATCH/PUT /scores/1
   def update
-    if @match_score.update(match_score_params)
-      UpdateGroupsGroupScoresAndSave.call(group: @match_score.match.group) if @match_score.part_of_group_match?
-      push_sync_if_needed!(@match_score.tournament)
-      TournamentLiveBroadcast.call(@match_score.tournament.id)
-      render json: @match_score
-    else
-      render json: @match_score.errors, status: :unprocessable_content
+    synced_tournament = nil
+
+    MatchScore.transaction do
+      tournament = @match_score.tournament
+      TournamentTransactionLogger.lock_tournament!(tournament)
+      @match_score.reload
+      old_values = score_values(@match_score)
+
+      if @match_score.update(match_score_params)
+        group_scores = []
+        if @match_score.part_of_group_match?
+          UpdateGroupsGroupScoresAndSave.call(group: @match_score.match.group)
+          group_scores = group_scores_for_log(@match_score.match.group)
+        end
+        TournamentTransactionLogger.record_match_score_update!(
+          match_score: @match_score,
+          user: current_user,
+          changes: score_changes(old_values, @match_score),
+          group_scores: group_scores
+        )
+        synced_tournament = tournament
+        render json: @match_score
+      else
+        render json: @match_score.errors, status: :unprocessable_content
+        raise ActiveRecord::Rollback
+      end
     end
+
+    return if synced_tournament.nil?
+
+    push_sync_if_needed!(synced_tournament)
+    TournamentLiveBroadcast.call(synced_tournament.id)
   end
 
   private
@@ -35,6 +59,30 @@ class MatchScoresController < ApplicationController
     params.slice(:points, :hidden_points).permit!
   end
 
+  def score_values(match_score)
+    {
+      points: match_score.points,
+      hidden_points: match_score.hidden_points
+    }
+  end
+
+  def score_changes(old_values, match_score)
+    permitted_params = match_score_params
+    score_values(match_score).each_with_object({}) do |(field, new_value), changes|
+      next unless permitted_params.key?(field) || permitted_params.key?(field.to_s)
+      next if old_values[field] == new_value
+
+      changes[field] = {
+        from: old_values[field],
+        to: new_value
+      }
+    end
+  end
+
+  def group_scores_for_log(group)
+    group.group_scores.includes(:team).reload
+  end
+
   def push_sync_if_needed!(tournament)
     TournamentSyncEnqueue.call(tournament)
   end
diff --git a/app/controllers/matches_controller.rb b/app/controllers/matches_controller.rb
index 8a83897..42477ba 100644
--- a/app/controllers/matches_controller.rb
+++ b/app/controllers/matches_controller.rb
@@ -23,37 +23,41 @@ class MatchesController < ApplicationController
 
   # POST /groups/:group_id/matches
   def create
-    match = build_decider_match
+    match = nil
+    synced_tournament = nil
 
-    if match.errors.any?
-      render json: match.errors, status: :unprocessable_content
-    elsif match.save
-      UpdateGroupsGroupScoresAndSave.call(group: @group)
-      push_sync_if_needed!(@group.tournament)
-      TournamentLiveBroadcast.call(@group.tournament.id)
-      render json: match, include: %w[match_scores.team group], status: :created
-    else
-      render json: match.errors, status: :unprocessable_content
+    Match.transaction do
+      TournamentTransactionLogger.lock_tournament!(@group.tournament)
+      match = build_decider_match
+
+      if match.errors.any?
+        render json: match.errors, status: :unprocessable_content
+        raise ActiveRecord::Rollback
+      elsif match.save
+        UpdateGroupsGroupScoresAndSave.call(group: @group)
+        group_scores = group_scores_for_log(@group)
+        TournamentTransactionLogger.record_decider_match_created!(match: match, user: current_user, group_scores: group_scores)
+        synced_tournament = @group.tournament
+        render json: match, include: %w[match_scores.team group], status: :created
+      else
+        render json: match.errors, status: :unprocessable_content
+        raise ActiveRecord::Rollback
+      end
     end
+
+    push_sync_if_needed!(synced_tournament) unless synced_tournament.nil?
+    TournamentLiveBroadcast.call(synced_tournament.id) unless synced_tournament.nil?
   end
 
   # PATCH/PUT /matches/1
   def update
     new_state = match_params['state']
     synced_tournament = nil
+    @transaction_log_group_scores = []
+    @transaction_log_advanced_match_ids = []
 
     Match.transaction do
-      if @match.update(match_params)
-        handle_match_end if new_state == 'finished'
-        handle_match_stop if new_state == 'not_started'
-        update_group_scores if @match.group_match? && %w[in_progress not_started].include?(new_state)
-
-        render json: @match
-        synced_tournament = @match.tournament
-      else
-        render json: @match.errors, status: :unprocessable_content
-        raise ActiveRecord::Rollback
-      end
+      synced_tournament = update_match_inside_transaction(new_state)
     end
     push_sync_if_needed!(synced_tournament) unless synced_tournament.nil?
     TournamentLiveBroadcast.call(synced_tournament.id) unless synced_tournament.nil?
@@ -92,17 +96,21 @@ class MatchesController < ApplicationController
       unless UpdateGroupsGroupScoresAndSave.call(group: group).success?
         logger.warn "Updating groups group score failed for #{group}"
       end
+      @transaction_log_group_scores = group_scores_for_log(group)
+      return
     end
 
-    return if @match.group_match?
-
     if @match.winner.nil?
       render json: { error: 'Stopping undecided Matches isn\'t allowed in playoff stage' },
              status: :unprocessable_content
       raise ActiveRecord::Rollback
     end
 
-    return if PopulateMatchBelowAndSave.call(match: @match).success?
+    result = PopulateMatchBelowAndSave.call(match: @match)
+    if result.success?
+      @transaction_log_advanced_match_ids = advanced_match_ids_for(result)
+      return
+    end
 
     render json: { error: 'Moving Team one stage down failed' },
            status: :unprocessable_content
@@ -117,27 +125,62 @@ class MatchesController < ApplicationController
 
   def update_group_scores
     group = @match.group
-    return if UpdateGroupsGroupScoresAndSave.call(group: group).success?
+    if UpdateGroupsGroupScoresAndSave.call(group: group).success?
+      @transaction_log_group_scores = group_scores_for_log(group)
+      return
+    end
 
     logger.warn "Updating groups group score failed for #{group}"
   end
 
-  def validate_params
-    case match_params['state']
-    when 'in_progress'
-      render json: { error: 'Match can\'t start in this state' }, status: :unprocessable_content \
-        unless @match.not_started?
-    when 'finished'
-      render json: { error: 'Match can\'t finish in this state' }, status: :unprocessable_content \
-        unless @match.in_progress?
-    when 'not_started'
-      render json: { error: 'Match can\'t stop in this state' }, status: :unprocessable_content \
-        unless @match.in_progress?
+  def update_match_inside_transaction(new_state)
+    tournament = @match.tournament
+    TournamentTransactionLogger.lock_tournament!(tournament)
+    @match.reload
+    validate_transition_inside_transaction(new_state)
+
+    old_state = @match.state
+    if @match.update(match_params)
+      record_successful_match_update(old_state, new_state)
+      render json: @match
+      tournament
     else
-      render json: { error: 'Invalid target state' }, status: :unprocessable_content
+      render json: @match.errors, status: :unprocessable_content
+      raise ActiveRecord::Rollback
     end
   end
 
+  def validate_transition_inside_transaction(new_state)
+    transition_error = match_transition_error(@match, new_state)
+    return if transition_error.nil?
+
+    render json: { error: transition_error }, status: :unprocessable_content
+    raise ActiveRecord::Rollback
+  end
+
+  def record_successful_match_update(old_state, new_state)
+    handle_match_end if new_state == 'finished'
+    handle_match_stop if new_state == 'not_started'
+    update_group_scores if @match.group_match? && %w[in_progress not_started].include?(new_state)
+    TournamentTransactionLogger.record_match_state_change!(
+      match: @match,
+      user: current_user,
+      old_state: old_state,
+      new_state: new_state,
+      details: {
+        group_scores: @transaction_log_group_scores,
+        advanced_match_ids: @transaction_log_advanced_match_ids
+      }
+    )
+  end
+
+  def validate_params
+    transition_error = match_transition_error(@match, match_params['state'])
+    return if transition_error.nil?
+
+    render json: { error: transition_error }, status: :unprocessable_content
+  end
+
   def set_match
     @match = Match.find(params[:id])
   end
@@ -186,6 +229,27 @@ class MatchesController < ApplicationController
     end
   end
 
+  def match_transition_error(match, new_state)
+    case new_state
+    when 'in_progress'
+      'Match can\'t start in this state' unless match.not_started?
+    when 'finished'
+      'Match can\'t finish in this state' unless match.in_progress?
+    when 'not_started'
+      'Match can\'t stop in this state' unless match.in_progress?
+    else
+      'Invalid target state'
+    end
+  end
+
+  def group_scores_for_log(group)
+    group.group_scores.includes(:team).reload
+  end
+
+  def advanced_match_ids_for(result)
+    Array(result.object_to_save).flatten.grep(Match).reject { |match| match.id == @match.id }.map(&:id)
+  end
+
   def push_sync_if_needed!(tournament)
     TournamentSyncEnqueue.call(tournament)
   end
diff --git a/app/controllers/stages_controller.rb b/app/controllers/stages_controller.rb
index 972b159..194bbc0 100644
--- a/app/controllers/stages_controller.rb
+++ b/app/controllers/stages_controller.rb
@@ -13,43 +13,81 @@ class StagesController < ApplicationController
 
   # PUT /stages/1
   def update
-    if stage_params[:state] == 'finished'
-      unless @stage.state == 'in_progress'
-        render json: { error: 'Only running group stages can be finished' }, status: :unprocessable_content
-        return
-      end
+    return render_invalid_stage_state unless stage_params[:state] == 'finished'
+    return render_stage_not_running unless @stage.in_progress?
 
-      synced_tournament = nil
-      Stage.transaction do
-        if @stage.update(stage_params)
-          handle_group_stage_end
-
-          render json: @stage
-          synced_tournament = @stage.tournament
-        else
-          render json: @stage.errors, status: :unprocessable_content
-          raise ActiveRecord::Rollback
-        end
-      end
-      push_sync_if_needed!(synced_tournament) unless synced_tournament.nil?
-      TournamentLiveBroadcast.call(synced_tournament.id) unless synced_tournament.nil?
-    else
-      render json: {
-        error: 'The state attribute may only be changed to finished'
-      }, status: :unprocessable_content
-    end
+    finish_group_stage
   end
 
   private
 
-  def handle_group_stage_end
-    unless @stage.over?
-      render json: {
-        error: 'Group Stage still has some matches that are not over yet. Finish them to generate playoffs'
-      }, status: :unprocessable_content
+  def finish_group_stage
+    synced_tournament = nil
+    @playoff_start_failure = nil
+    @transaction_log_advancing_teams = []
+
+    Stage.transaction do
+      synced_tournament = finish_group_stage_inside_transaction
+    end
+    record_playoff_start_failure if @playoff_start_failure
+    push_sync_if_needed!(synced_tournament) unless synced_tournament.nil?
+    TournamentLiveBroadcast.call(synced_tournament.id) unless synced_tournament.nil?
+  end
+
+  def finish_group_stage_inside_transaction
+    TournamentTransactionLogger.lock_tournament!(@stage.tournament)
+    @stage.reload
+    return unless stage_running_for_playoff_start?
+    return unless group_stage_over_for_playoff_start?
+    return unless recalculate_group_scores_for_playoff_start
+    return unless no_blocking_ties_for_playoff_start?
+
+    start_playoffs_inside_transaction
+  end
+
+  def start_playoffs_inside_transaction
+    @transaction_log_advancing_teams = GroupStageService.get_advancing_teams(@stage)
+    if @stage.update(stage_params) &&
+       AddPlayoffsToTournamentAndSave.call(tournament: @stage.tournament,
+                                           teams: @transaction_log_advancing_teams).success?
+      record_playoffs_started
+      render json: @stage
+      return @stage.tournament
+    elsif @stage.errors.any?
+      render json: @stage.errors, status: :unprocessable_content
       raise ActiveRecord::Rollback
     end
 
+    @playoff_start_failure = { reason: 'generation_failed', blocking_ties: [] }
+    render json: { error: 'Generating group stage failed' }, status: :unprocessable_content
+    raise ActiveRecord::Rollback
+  end
+
+  def render_invalid_stage_state
+    render json: {
+      error: 'The state attribute may only be changed to finished'
+    }, status: :unprocessable_content
+  end
+
+  def render_stage_not_running
+    render json: { error: 'Only running group stages can be finished' }, status: :unprocessable_content
+  end
+
+  def stage_running_for_playoff_start?
+    return true if @stage.in_progress?
+
+    render_stage_not_running
+    false
+  end
+
+  def group_stage_over_for_playoff_start?
+    return true if @stage.over?
+
+    render_playoff_start_failure(reason: 'unfinished_matches')
+    false
+  end
+
+  def recalculate_group_scores_for_playoff_start
     @stage.groups.each do |group|
       next if UpdateGroupsGroupScoresAndSave.call(group: group).success?
 
@@ -57,20 +95,56 @@ class StagesController < ApplicationController
       raise ActiveRecord::Rollback
     end
 
+    true
+  end
+
+  def no_blocking_ties_for_playoff_start?
     blocking_ties = GroupStageService.blocking_ties_for(@stage)
-    if blocking_ties.any?
-      render json: {
-        error: 'Playoffs cannot be generated yet because a decider match is required for an advancing place',
-        blocking_ties: blocking_ties
-      }, status: :unprocessable_content
-      raise ActiveRecord::Rollback
+    return true if blocking_ties.empty?
+
+    render_playoff_start_failure(reason: 'decider_missing', blocking_ties: blocking_ties)
+    false
+  end
+
+  def render_playoff_start_failure(reason:, blocking_ties: [])
+    TournamentTransactionLogger.record_playoffs_start_failed!(
+      stage: @stage,
+      user: current_user,
+      reason: reason,
+      blocking_ties: blocking_ties
+    )
+    render json: playoff_start_failure_payload(reason, blocking_ties), status: :unprocessable_content
+    nil
+  end
+
+  def playoff_start_failure_payload(reason, blocking_ties)
+    if reason == 'unfinished_matches'
+      return {
+        error: 'Group Stage still has some matches that are not over yet. Finish them to generate playoffs'
+      }
     end
 
-    return if AddPlayoffsToTournamentAndSave.call(tournament: @stage.tournament,
-                                                  teams: GroupStageService.get_advancing_teams(@stage)).success?
+    {
+      error: 'Playoffs cannot be generated yet because a decider match is required for an advancing place',
+      blocking_ties: blocking_ties
+    }
+  end
 
-    render json: { error: 'Generating group stage failed' }, status: :unprocessable_content
-    raise ActiveRecord::Rollback
+  def record_playoffs_started
+    TournamentTransactionLogger.record_playoffs_started!(
+      stage: @stage,
+      user: current_user,
+      advancing_teams: @transaction_log_advancing_teams
+    )
+  end
+
+  def record_playoff_start_failure
+    TournamentTransactionLogger.record_playoffs_start_failed!(
+      stage: @stage,
+      user: current_user,
+      reason: @playoff_start_failure.fetch(:reason),
+      blocking_ties: @playoff_start_failure.fetch(:blocking_ties)
+    )
   end
 
   def set_stage
diff --git a/app/controllers/tournament_transaction_log_entries_controller.rb b/app/controllers/tournament_transaction_log_entries_controller.rb
new file mode 100644
index 0000000..d860396
--- /dev/null
+++ b/app/controllers/tournament_transaction_log_entries_controller.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class TournamentTransactionLogEntriesController < ApplicationController
+  before_action :set_tournament
+  before_action :authenticate_user!
+  before_action -> { require_owner! @tournament.owner }
+
+  def index
+    entries = @tournament.transaction_log_entries.includes(:user).order(:sequence)
+    render json: entries, each_serializer: TournamentTransactionLogEntrySerializer
+  end
+
+  private
+
+  def set_tournament
+    @tournament = Tournament.find(params[:id])
+  end
+end
diff --git a/app/controllers/tournaments_controller.rb b/app/controllers/tournaments_controller.rb
index 6c08577..f878c77 100644
--- a/app/controllers/tournaments_controller.rb
+++ b/app/controllers/tournaments_controller.rb
@@ -65,6 +65,7 @@ class TournamentsController < ApplicationController
 
     if tournament.read_only_mode?
       if tournament.save
+        TournamentTransactionLogger.record_tournament_created!(tournament: tournament, user: current_user)
         render json: tournament, status: :created, location: tournament
       else
         render json: tournament.errors, status: :unprocessable_content
@@ -95,6 +96,7 @@ class TournamentsController < ApplicationController
     end
     # return appropriate result
     if result.success?
+      TournamentTransactionLogger.record_tournament_created!(tournament: result.tournament, user: current_user)
       render json: result.tournament, status: :created, location: result.tournament
     else
       render json: { error: 'Tournament generation failed' }, status: :unprocessable_content
@@ -135,12 +137,29 @@ class TournamentsController < ApplicationController
 
   # PATCH /tournaments/:id/timer
   def set_timer
-    if @tournament.update(timer_params)
-      push_sync_if_needed!(@tournament)
-      render json: @tournament
-    else
-      render json: @tournament.errors, status: :unprocessable_content
+    synced_tournament = nil
+
+    Tournament.transaction do
+      TournamentTransactionLogger.lock_tournament!(@tournament)
+      @tournament.reload
+      old_timer = serialize_timer_state(@tournament)
+
+      if @tournament.update(timer_params)
+        TournamentTransactionLogger.record_timer_changed!(
+          tournament: @tournament,
+          user: current_user,
+          old_timer: old_timer,
+          new_timer: serialize_timer_state(@tournament)
+        )
+        synced_tournament = @tournament
+        render json: @tournament
+      else
+        render json: @tournament.errors, status: :unprocessable_content
+        raise ActiveRecord::Rollback
+      end
     end
+
+    push_sync_if_needed!(synced_tournament) unless synced_tournament.nil?
   end
 
   def sync_state
@@ -170,7 +189,7 @@ class TournamentsController < ApplicationController
 
   # PATCH /tournaments/:id/start_next_group_matches
   def start_next_group_matches
-    started_matches = TournamentGroupMatchStarter.start_next!(@tournament, logger:)
+    started_matches = TournamentGroupMatchStarter.start_next!(@tournament, logger:, user: current_user)
     if started_matches.any?
       push_sync_if_needed!(@tournament)
       TournamentLiveBroadcast.call(@tournament.id)
diff --git a/app/models/tournament.rb b/app/models/tournament.rb
index 0daf7ae..4063d27 100644
--- a/app/models/tournament.rb
+++ b/app/models/tournament.rb
@@ -13,6 +13,7 @@ class Tournament < ApplicationRecord
   has_many :stages, dependent: :destroy
   has_many :beamers, dependent: :destroy
   has_many :team_action_lists, dependent: :destroy
+  has_many :transaction_log_entries, class_name: 'TournamentTransactionLogEntry', dependent: :destroy
 
   validates :name, presence: true
   validates :code, presence: true, uniqueness: true
diff --git a/app/models/tournament_transaction_log_entry.rb b/app/models/tournament_transaction_log_entry.rb
new file mode 100644
index 0000000..88128f1
--- /dev/null
+++ b/app/models/tournament_transaction_log_entry.rb
@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class TournamentTransactionLogEntry < ApplicationRecord
+  belongs_to :tournament
+  belongs_to :user, optional: true
+
+  validates :sequence, presence: true, numericality: { only_integer: true, greater_than: 0 }
+  validates :action, presence: true
+  validates :message, presence: true
+end
diff --git a/app/serializers/tournament_transaction_log_entry_serializer.rb b/app/serializers/tournament_transaction_log_entry_serializer.rb
new file mode 100644
index 0000000..aa8f708
--- /dev/null
+++ b/app/serializers/tournament_transaction_log_entry_serializer.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class TournamentTransactionLogEntrySerializer < ApplicationSerializer
+  attributes :id, :sequence, :action, :message, :metadata
+
+  attribute :created_at do
+    object.created_at&.iso8601
+  end
+
+  attribute :user do
+    next nil if object.user.nil?
+
+    {
+      id: object.user.id,
+      username: object.user.username
+    }
+  end
+end
diff --git a/app/services/tournament_group_match_starter.rb b/app/services/tournament_group_match_starter.rb
index a6575c8..f11db45 100644
--- a/app/services/tournament_group_match_starter.rb
+++ b/app/services/tournament_group_match_starter.rb
@@ -10,13 +10,19 @@ class TournamentGroupMatchStarter
       selected_lowest_matches(group_matches, lowest_position)
     end
 
-    def start_next!(tournament, logger: Rails.logger)
+    def start_next!(tournament, logger: Rails.logger, user: nil)
       started_matches = []
 
       Match.transaction do
+        TournamentTransactionLogger.lock_tournament!(tournament)
         started_matches = upcoming_matches(tournament)
         started_matches.each { |match| match.update!(state: :in_progress) }
         recalculate_group_scores!(started_matches, logger:)
+        TournamentTransactionLogger.record_group_matches_started!(
+          tournament: tournament,
+          user: user,
+          matches: started_matches
+        )
       end
 
       started_matches
diff --git a/app/services/tournament_transaction_logger.rb b/app/services/tournament_transaction_logger.rb
new file mode 100644
index 0000000..90b0255
--- /dev/null
+++ b/app/services/tournament_transaction_logger.rb
@@ -0,0 +1,347 @@
+# frozen_string_literal: true
+
+class TournamentTransactionLogger
+  class << self
+    def lock_tournament!(tournament)
+      raise ArgumentError, 'tournament is required' if tournament.nil?
+
+      unless ActiveRecord::Base.connection.transaction_open?
+        raise 'Tournament transaction log writes must happen inside a transaction'
+      end
+
+      # Lock a fresh row instance so callers keep saved_change_to_* state for
+      # after_commit callbacks on the object they just updated.
+      Tournament.lock.find(tournament.id)
+    end
+
+    def record!(tournament:, user:, action:, message:, metadata: {})
+      retries = 0
+
+      begin
+        Tournament.transaction(requires_new: true) do
+          lock_tournament!(tournament)
+          TournamentTransactionLogEntry.create!(
+            tournament: tournament,
+            user: user,
+            sequence: next_sequence_for(tournament),
+            action: action,
+            message: message,
+            metadata: metadata.deep_stringify_keys
+          )
+        end
+      rescue ActiveRecord::RecordNotUnique
+        retries += 1
+        retry if retries < 3
+
+        raise
+      end
+    end
+
+    def record_tournament_created!(tournament:, user:)
+      record!(
+        tournament: tournament,
+        user: user,
+        action: 'tournament.created',
+        message: "#{user_label(user)} created tournament #{tournament.name}(#{tournament.id})",
+        metadata: {
+          tournament_id: tournament.id,
+          tournament_name: tournament.name
+        }
+      )
+    end
+
+    def record_timer_changed!(tournament:, user:, old_timer:, new_timer:)
+      record!(
+        tournament: tournament,
+        user: user,
+        action: 'tournament.timer_changed',
+        message: "#{user_label(user)} changed tournament timer for #{tournament.name}(#{tournament.id})",
+        metadata: {
+          old_timer: old_timer,
+          new_timer: new_timer
+        }
+      )
+    end
+
+    def record_match_state_change!(match:, user:, old_state:, new_state:, details: {})
+      match.reload
+      stage_type = match.group_match? ? 'groupstage' : 'playoffs'
+      group_scores = details.fetch(:group_scores, [])
+      advanced_match_ids = details.fetch(:advanced_match_ids, [])
+      metadata = match_metadata(match).merge(
+        old_state: old_state,
+        new_state: new_state,
+        stage_type: stage_type,
+        winner_team_id: match.winner&.id,
+        advanced_match_ids: advanced_match_ids,
+        group_scores: group_scores_metadata(group_scores)
+      )
+
+      record!(
+        tournament: match.tournament,
+        user: user,
+        action: 'match.state_changed',
+        message: match_state_message(
+          match: match,
+          user: user,
+          new_state: new_state,
+          details: details.merge(stage_type: stage_type)
+        ),
+        metadata: metadata
+      )
+    end
+
+    def record_match_score_update!(match_score:, user:, changes:, group_scores: [])
+      return if changes.empty?
+
+      match_score.reload
+      match = match_score.match
+      metadata = match_metadata(match).merge(
+        match_score_id: match_score.id,
+        team_id: match_score.team_id,
+        changes: changes,
+        group_scores: group_scores_metadata(group_scores)
+      )
+
+      record!(
+        tournament: match.tournament,
+        user: user,
+        action: 'match_score.updated',
+        message: match_score_message(
+          match_score: match_score,
+          user: user,
+          changes: changes,
+          group_scores: group_scores
+        ),
+        metadata: metadata
+      )
+    end
+
+    def record_decider_match_created!(match:, user:, group_scores: [])
+      record!(
+        tournament: match.tournament,
+        user: user,
+        action: 'decider_match.created',
+        message: "#{user_label(user)} added decider match #{match.id} (#{match_teams_label(match)})",
+        metadata: match_metadata(match).merge(group_scores: group_scores_metadata(group_scores))
+      )
+    end
+
+    def record_playoffs_start_failed!(stage:, user:, reason:, blocking_ties: [])
+      record!(
+        tournament: stage.tournament,
+        user: user,
+        action: 'playoffs.start_failed',
+        message: playoffs_start_failed_message(user: user, reason: reason, blocking_ties: blocking_ties),
+        metadata: {
+          stage_id: stage.id,
+          reason: reason,
+          blocking_ties: blocking_ties
+        }
+      )
+    end
+
+    def record_playoffs_started!(stage:, user:, advancing_teams:)
+      record!(
+        tournament: stage.tournament,
+        user: user,
+        action: 'playoffs.started',
+        message: "#{user_label(user)} started playoffs, advancing teams: #{team_list_label(advancing_teams)}",
+        metadata: {
+          stage_id: stage.id,
+          advancing_team_ids: advancing_teams.map(&:id),
+          advancing_teams: teams_metadata(advancing_teams)
+        }
+      )
+    end
+
+    def record_group_matches_started!(tournament:, user:, matches:)
+      return if matches.empty?
+
+      record!(
+        tournament: tournament,
+        user: user,
+        action: 'group_matches.started',
+        message: "#{user_label(user)} started group matches: #{matches.map(&:id).join(', ')}",
+        metadata: {
+          match_ids: matches.map(&:id),
+          matches: matches.map { |match| match_metadata(match) }
+        }
+      )
+    end
+
+    private
+
+    def next_sequence_for(tournament)
+      TournamentTransactionLogEntry.where(tournament_id: tournament.id).maximum(:sequence).to_i + 1
+    end
+
+    def user_label(user)
+      user&.username.presence || user&.email.presence || 'system'
+    end
+
+    def state_label(state)
+      {
+        'in_progress' => 'running',
+        'not_started' => 'not started',
+        'not_ready' => 'not ready',
+        'single_team' => 'single team',
+        'undecided' => 'undecided',
+        'finished' => 'finished'
+      }.fetch(state.to_s, state.to_s)
+    end
+
+    def match_state_message(match:, user:, new_state:, details:)
+      group_scores = details.fetch(:group_scores, [])
+      advanced_match_ids = details.fetch(:advanced_match_ids, [])
+      stage_type = details.fetch(:stage_type)
+
+      if new_state.to_s == 'finished'
+        return finished_group_match_message(match, user, group_scores) if match.group_match?
+
+        return finished_playoff_match_message(match, user, stage_type, advanced_match_ids)
+      end
+
+      message = "#{user_label(user)} changed match #{match.id} " \
+                "(#{match_teams_label(match)}) to #{state_label(new_state)}"
+      message += ' - scores reset' if new_state.to_s == 'not_started'
+      message
+    end
+
+    def finished_group_match_message(match, user, group_scores)
+      "#{user_label(user)} changed match #{match.id} to finished (#{finish_result_label(match)}) - " \
+        "groupstage - group scores recalculated (#{group_scores_label(group_scores)})"
+    end
+
+    def finished_playoff_match_message(match, user, stage_type, advanced_match_ids)
+      message = "#{user_label(user)} changed match #{match.id} to finished " \
+                "(#{finish_result_label(match)}) - #{stage_type}"
+      return message if advanced_match_ids.empty? || match.winner.nil?
+
+      "#{message} - #{team_label(match.winner)} advances to match #{advanced_match_ids.join(', ')}"
+    end
+
+    def match_score_message(match_score:, user:, changes:, group_scores:)
+      match = match_score.match
+      change_label = changes.map do |field, values|
+        "#{field} from #{values.fetch(:from)} to #{values.fetch(:to)}"
+      end.join(', ')
+      message = "#{user_label(user)} changed matchscore #{match_score.id} from match #{match.id}: #{change_label} " \
+                "(#{score_detail_label(match_score, changes)})"
+      return message unless match.group_match?
+
+      "#{message} - group scores recalculated (#{group_scores_label(group_scores)})"
+    end
+
+    def playoffs_start_failed_message(user:, reason:, blocking_ties:)
+      case reason.to_s
+      when 'decider_missing'
+        "#{user_label(user)} attempted playoff start, failed because decider missing " \
+          "(#{blocking_ties_label(blocking_ties)})"
+      when 'unfinished_matches'
+        "#{user_label(user)} attempted playoff start, failed because group stage has unfinished matches"
+      else
+        "#{user_label(user)} attempted playoff start, failed because #{reason}"
+      end
+    end
+
+    def finish_result_label(match)
+      winner = match.winner
+      return "#{team_label(winner)} wins" unless winner.nil?
+
+      teams = match.teams
+      return "draw between #{team_label(teams.first)} and #{team_label(teams.second)}" if teams.size == 2
+
+      'no winner'
+    end
+
+    def score_detail_label(match_score, changes)
+      team = match_score.team
+      opponent = match_score.match.teams.find { |candidate| candidate.id != team.id }
+      field = score_detail_field(changes)
+      points = score_detail_points(match_score, changes, field)
+      "#{points} #{field.to_s.tr('_', ' ')} for #{team_label(team)} in match vs #{team_label(opponent)}"
+    end
+
+    def score_detail_field(changes)
+      return :hidden_points if changes.key?(:hidden_points) || changes.key?('hidden_points')
+
+      :points
+    end
+
+    def score_detail_points(match_score, changes, field)
+      changes.dig(field.to_s, :to) ||
+        changes.dig(field, :to) ||
+        match_score.public_send(field)
+    end
+
+    def match_teams_label(match)
+      teams = match.teams
+      return "between #{team_label(teams.first)} and #{team_label(teams.second)}" if teams.size == 2
+      return "with #{team_label(teams.first)}" if teams.size == 1
+
+      'without assigned teams'
+    end
+
+    def team_label(team)
+      return 'unknown team' if team.nil?
+
+      "#{team.name}(#{team.id})"
+    end
+
+    def team_list_label(teams)
+      teams.map { |team| team_label(team) }.join(', ')
+    end
+
+    def blocking_ties_label(blocking_ties)
+      blocking_ties.map do |tie|
+        names = tie.fetch(:team_names, [])
+        ids = tie.fetch(:team_ids, [])
+        names.zip(ids).map { |name, id| "#{name}(#{id})" }.join(' vs ')
+      end.join(', ')
+    end
+
+    def group_scores_label(group_scores)
+      group_scores_metadata(group_scores).map do |score|
+        "#{score.fetch(:team_name)}(#{score.fetch(:team_id)}) groupscore " \
+          "#{score.fetch(:group_points)}-#{score.fetch(:scored_points)}-#{score.fetch(:received_points)} " \
+          "pos #{score.fetch(:position)}"
+      end.join(', ')
+    end
+
+    def group_scores_metadata(group_scores)
+      Array(group_scores).map do |score|
+        score.reload if score.persisted?
+        {
+          id: score.id,
+          team_id: score.team_id,
+          team_name: score.team.name,
+          group_points: score.group_points,
+          scored_points: score.scored_points,
+          received_points: score.received_points,
+          position: score.position
+        }
+      end
+    end
+
+    def match_metadata(match)
+      {
+        match_id: match.id,
+        decider: match.decider?,
+        group_id: match.group_id,
+        stage_id: match.stage_id,
+        team_ids: match.teams.map(&:id),
+        teams: teams_metadata(match.teams)
+      }
+    end
+
+    def teams_metadata(teams)
+      teams.map do |team|
+        {
+          id: team.id,
+          name: team.name
+        }
+      end
+    end
+  end
+end
diff --git a/config/routes.rb b/config/routes.rb
index ce2aeb8..3176d07 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -32,6 +32,7 @@ Rails.application.routes.draw do
     end
     member do
       get :timer
+      get :transaction_log, to: 'tournament_transaction_log_entries#index'
       patch :timer, action: :set_timer
       patch :sync_state
       patch :start_next_group_matches
diff --git a/db/migrate/20260430121000_create_tournament_transaction_log_entries.rb b/db/migrate/20260430121000_create_tournament_transaction_log_entries.rb
new file mode 100644
index 0000000..ed194c8
--- /dev/null
+++ b/db/migrate/20260430121000_create_tournament_transaction_log_entries.rb
@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class CreateTournamentTransactionLogEntries < ActiveRecord::Migration[7.0]
+  def change
+    create_table :tournament_transaction_log_entries do |t|
+      t.references :tournament, null: false, foreign_key: { on_delete: :cascade }, index: true
+      t.references :user, foreign_key: { on_delete: :nullify }, index: true
+      t.integer :sequence, null: false
+      t.string :action, null: false
+      t.text :message, null: false
+      t.json :metadata, null: false, default: {}
+
+      t.timestamps
+    end
+
+    add_index :tournament_transaction_log_entries,
+              [:tournament_id, :sequence],
+              unique: true,
+              name: 'index_tournament_log_entries_on_tournament_and_sequence'
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 262dbe1..773e654 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2026_04_30_120000) do
+ActiveRecord::Schema[8.1].define(version: 2026_04_30_121000) do
   create_table "beamers", force: :cascade do |t|
     t.json "config", default: {}, null: false
     t.datetime "created_at", null: false
@@ -147,6 +147,20 @@ ActiveRecord::Schema[8.1].define(version: 2026_04_30_120000) do
     t.index ["tournament_id"], name: "index_tournament_sync_queue_entries_on_tournament_id", unique: true
   end
 
+  create_table "tournament_transaction_log_entries", force: :cascade do |t|
+    t.string "action", null: false
+    t.datetime "created_at", null: false
+    t.text "message", null: false
+    t.json "metadata", default: {}, null: false
+    t.integer "sequence", null: false
+    t.integer "tournament_id", null: false
+    t.datetime "updated_at", null: false
+    t.integer "user_id"
+    t.index ["tournament_id", "sequence"], name: "index_tournament_log_entries_on_tournament_and_sequence", unique: true
+    t.index ["tournament_id"], name: "index_tournament_transaction_log_entries_on_tournament_id"
+    t.index ["user_id"], name: "index_tournament_transaction_log_entries_on_user_id"
+  end
+
   create_table "tournaments", force: :cascade do |t|
     t.string "code", null: false
     t.datetime "created_at", precision: nil, null: false
@@ -221,5 +235,7 @@ ActiveRecord::Schema[8.1].define(version: 2026_04_30_120000) do
   add_foreign_key "team_action_lists", "tournaments", on_delete: :cascade
   add_foreign_key "teams", "tournaments", on_delete: :cascade
   add_foreign_key "tournament_sync_queue_entries", "tournaments", on_delete: :cascade
+  add_foreign_key "tournament_transaction_log_entries", "tournaments", on_delete: :cascade
+  add_foreign_key "tournament_transaction_log_entries", "users", on_delete: :nullify
   add_foreign_key "tournaments", "users", on_delete: :cascade
 end
diff --git a/doc/tournament_transaction_log_api.md b/doc/tournament_transaction_log_api.md
new file mode 100644
index 0000000..b5aa0d7
--- /dev/null
+++ b/doc/tournament_transaction_log_api.md
@@ -0,0 +1,281 @@
+# Tournament Transaction Log API
+
+## Purpose
+
+Tournament transaction log is owner-readable tournament history.
+
+It records ordered tournament events such as:
+
+- tournament creation
+- timer changes
+- match state changes
+- match score changes
+- decider match creation
+- playoff start attempts and failures
+- playoff start success
+- group match auto-starts
+
+Frontend should show this as an append-only history feed for tournament owners.
+
+## Read Flow
+
+Transaction log is not embedded in normal tournament payload.
+Read it from dedicated owner-only endpoint:
+
+- `GET /tournaments/:id/transaction_log`
+
+Auth:
+
+- same Devise token auth headers as other owner APIs
+- endpoint requires authenticated tournament owner
+
+Response:
+
+- `200 OK`
+- JSON array
+- ordered by `sequence` ascending
+- no pagination yet
+
+Example:
+
+```json
+[
+  {
+    "id": 17,
+    "sequence": 1,
+    "action": "tournament.created",
+    "message": "owner created tournament Spring Cup(123)",
+    "metadata": {
+      "tournament_id": 123,
+      "tournament_name": "Spring Cup"
+    },
+    "created_at": "2026-04-30T10:12:42Z",
+    "user": {
+      "id": 4,
+      "username": "owner"
+    }
+  },
+  {
+    "id": 18,
+    "sequence": 2,
+    "action": "match.state_changed",
+    "message": "owner changed match 77 (Team A(10) vs Team B(11)) to running",
+    "metadata": {
+      "match_id": 77,
+      "stage_type": "playoffs",
+      "old_state": "not_started",
+      "new_state": "in_progress",
+      "team_ids": [10, 11],
+      "teams": [
+        { "id": 10, "name": "Team A" },
+        { "id": 11, "name": "Team B" }
+      ],
+      "winner_team_id": null,
+      "advanced_match_ids": [],
+      "group_scores": []
+    },
+    "created_at": "2026-04-30T10:13:01Z",
+    "user": {
+      "id": 4,
+      "username": "owner"
+    }
+  }
+]
+```
+
+## Field Contract
+
+Each entry has:
+
+- `id`: DB id for entry
+- `sequence`: per-tournament monotonic order number, starts at `1`
+- `action`: machine-readable action key
+- `message`: human-readable owner history text
+- `metadata`: structured action-specific data
+- `created_at`: ISO-8601 timestamp
+- `user`: actor object, or `null` for system actions
+
+`user` shape:
+
+```json
+{
+  "id": 4,
+  "username": "owner"
+}
+```
+
+Frontend display guidance:
+
+- sort by `sequence`, not by timestamp
+- use `message` for first UI pass
+- use `metadata` for links/details/chips
+- tolerate unknown future `action` values
+- tolerate extra `metadata` fields
+
+## Current Action Keys
+
+Current `action` values:
+
+- `tournament.created`
+- `tournament.timer_changed`
+- `match.state_changed`
+- `match_score.updated`
+- `decider_match.created`
+- `playoffs.start_failed`
+- `playoffs.started`
+- `group_matches.started`
+
+Future backend work may add more action keys.
+Unknown actions should still render via `message`.
+
+## Metadata Notes
+
+`metadata` is stable JSON, but shape depends on `action`.
+
+Common match metadata includes:
+
+- `match_id`
+- `stage_id`
+- `group_id`
+- `stage_type`: `groupstage` or `playoffs`
+- `team_ids`
+- `teams`
+- `winner_team_id`
+- `advanced_match_ids`
+- `group_scores`
+
+Team object shape:
+
+```json
+{
+  "id": 10,
+  "name": "Team A"
+}
+```
+
+Group score object shape:
+
+```json
+{
+  "team_id": 10,
+  "team_name": "Team A",
+  "points": 9,
+  "matches_played": 3,
+  "wins": 3,
+  "draws": 0,
+  "losses": 0,
+  "score": 12,
+  "score_against": 4,
+  "score_difference": 8,
+  "position": 1
+}
+```
+
+## Example Actions
+
+### Match state change
+
+```json
+{
+  "action": "match.state_changed",
+  "message": "owner changed match 77 to finished (Team A(10) wins) - playoffs - Team A(10) advances to match 80",
+  "metadata": {
+    "match_id": 77,
+    "stage_type": "playoffs",
+    "old_state": "in_progress",
+    "new_state": "finished",
+    "winner_team_id": 10,
+    "advanced_match_ids": [80]
+  }
+}
+```
+
+### Match score update
+
+```json
+{
+  "action": "match_score.updated",
+  "message": "owner changed matchscore 91 from match 77 to 3 (3 points for Team A(10) in match vs Team B(11))",
+  "metadata": {
+    "match_id": 77,
+    "match_score_id": 91,
+    "team_id": 10,
+    "changes": {
+      "points": {
+        "from": 2,
+        "to": 3
+      }
+    }
+  }
+}
+```
+
+### Playoff start failure
+
+```json
+{
+  "action": "playoffs.start_failed",
+  "message": "owner attempted playoff start, failed because decider missing",
+  "metadata": {
+    "stage_id": 12,
+    "reason": "decider_missing",
+    "blocking_ties": [
+      {
+        "group_id": 5,
+        "team_ids": [10, 11],
+        "teams": [
+          { "id": 10, "name": "Team A" },
+          { "id": 11, "name": "Team B" }
+        ]
+      }
+    ]
+  }
+}
+```
+
+### Playoffs started
+
+```json
+{
+  "action": "playoffs.started",
+  "message": "owner started playoffs, advancing teams: Team A(10), Team B(11), Team C(12), Team D(13)",
+  "metadata": {
+    "stage_id": 12,
+    "advancing_team_ids": [10, 11, 12, 13],
+    "advancing_teams": [
+      { "id": 10, "name": "Team A" },
+      { "id": 11, "name": "Team B" },
+      { "id": 12, "name": "Team C" },
+      { "id": 13, "name": "Team D" }
+    ]
+  }
+}
+```
+
+## Error Behavior
+
+Expected errors:
+
+- `401 Unauthorized`: missing or invalid auth
+- `403 Forbidden`: authenticated user is not tournament owner
+- `404 Not Found`: tournament id does not exist
+
+Read-only follower tournaments:
+
+- log read is allowed for owner
+- log write is internal only; frontend has no write endpoint
+
+## Frontend Implementation Guide
+
+Recommended owner-history flow:
+
+1. Load tournament as usual.
+2. If current user is owner, request `GET /tournaments/:id/transaction_log`.
+3. Render entries sorted by `sequence`.
+4. Use `message` as main text.
+5. Show `created_at` as event time.
+6. Link matches/teams from `metadata` when ids are present.
+7. Refresh after owner mutations, or poll while history panel is open.
+
+There is no websocket stream for transaction log yet.
+Existing tournament/match websocket updates still fire for live tournament state.
diff --git a/e2e/lib/scenario_runner.rb b/e2e/lib/scenario_runner.rb
index f3d3eb5..dace468 100644
--- a/e2e/lib/scenario_runner.rb
+++ b/e2e/lib/scenario_runner.rb
@@ -148,6 +148,58 @@ module TurniereE2E
       result(session: session, tournament: tournament, checkpoints: checkpoints, stop_at: stop_at)
     end
 
+    def run_transaction_log_history(stop_at: :transaction_log)
+      session = register_session!
+      tournament = create_tournament!(
+        create_group_stage_payload(group_count: 1, teams_per_group: 2, playoff_teams_amount: 1)
+      )
+
+      checkpoints = {
+        created: summarize_tournament(tournament)
+      }
+
+      group_stage = group_stage_of!(tournament)
+      group = group_stage.fetch(:groups).first
+      match = group.fetch(:matches).first
+      team_ids = match.fetch(:match_scores).map { |score| score.dig(:team, :id) }
+
+      play_match_with_points!(
+        match,
+        points_by_team_name: {
+          'G1-S1' => 1,
+          'G1-S2' => 1
+        }
+      )
+      checkpoints[:drawn_group_match] = fetch_match!(match.fetch(:id))
+
+      failed_playoff_start = client.patch("/stages/#{group_stage.fetch(:id)}", body: { state: 'finished' })
+      expect_status!(failed_playoff_start, 422, 'finish group stage without decider')
+      checkpoints[:failed_playoff_start] = failed_playoff_start.fetch(:json)
+
+      decider = create_decider_match!(group.fetch(:id), team_ids)
+      play_match_with_hidden_points!(
+        decider,
+        hidden_points_by_team_name: {
+          'G1-S1' => 2,
+          'G1-S2' => 1
+        }
+      )
+      checkpoints[:decider_finished] = fetch_match!(decider.fetch(:id))
+
+      finish_stage!(group_stage.fetch(:id))
+      tournament = fetch_tournament!(tournament.fetch(:id))
+      checkpoints[:playoffs_created] = summarize_tournament(tournament)
+
+      transaction_log = get!("/tournaments/#{tournament.fetch(:id)}/transaction_log")
+      checkpoints[:transaction_log] = {
+        actions: transaction_log.map { |entry| entry.fetch(:action) },
+        sequences: transaction_log.map { |entry| entry.fetch(:sequence) },
+        messages: transaction_log.map { |entry| entry.fetch(:message) }
+      }
+
+      result(session: session, tournament: tournament, checkpoints: checkpoints, stop_at: stop_at)
+    end
+
     def run_group_stage_render_profile(group_count:, teams_per_group:, playoff_teams_amount:, stop_at: :profiled_show)
       session = register_session!
       tournament = create_tournament!(
@@ -339,6 +391,36 @@ module TurniereE2E
       }
     end
 
+    def play_match_with_hidden_points!(match, hidden_points_by_team_name:, expected_finish_status: 200)
+      started = client.patch("/matches/#{match.fetch(:id)}", body: { state: 'in_progress' })
+      expect_status!(started, 200, "start match #{match.fetch(:id)}")
+
+      refreshed_match = fetch_match!(match.fetch(:id))
+      refreshed_match.fetch(:match_scores).each do |score|
+        team_name = score.dig(:team, :name)
+        hidden_points = hidden_points_by_team_name.fetch(team_name) do
+          raise "missing hidden score for #{team_name.inspect} in match #{match.fetch(:id)}"
+        end
+        updated_score = client.patch("/match_scores/#{score.fetch(:id)}", body: { hidden_points: hidden_points })
+        expect_status!(updated_score, 200, "update hidden match_score #{score.fetch(:id)}")
+      end
+
+      finished = client.patch("/matches/#{match.fetch(:id)}", body: { state: 'finished' })
+      expect_status!(finished, expected_finish_status, "finish match #{match.fetch(:id)}")
+
+      {
+        match_id: match.fetch(:id),
+        status: finished.fetch(:status),
+        body: finished.fetch(:json)
+      }
+    end
+
+    def create_decider_match!(group_id, team_ids)
+      response = client.post("/groups/#{group_id}/matches", body: { team_ids: team_ids })
+      expect_status!(response, 201, "create decider match for group #{group_id}")
+      response.fetch(:json)
+    end
+
     def fetch_match!(match_id)
       response = client.get("/matches/#{match_id}")
       expect_status!(response, 200, "fetch match #{match_id}")
diff --git a/spec/controllers/match_scores_controller_spec.rb b/spec/controllers/match_scores_controller_spec.rb
index 4a23db8..94f1fc4 100644
--- a/spec/controllers/match_scores_controller_spec.rb
+++ b/spec/controllers/match_scores_controller_spec.rb
@@ -51,6 +51,23 @@ RSpec.describe MatchScoresController, type: :controller do
             expect(body[:points]).to eq(valid_update[:points])
             expect(body[:hidden_points]).to eq(valid_update[:hidden_points])
           end
+
+          it 'writes a transaction log entry for the score change' do
+            expect(TournamentLiveBroadcast).to receive(:call).with(@match_score.tournament.id)
+
+            expect do
+              put :update, params: { id: @match_score.to_param }.merge(valid_update)
+            end.to change { @match_score.tournament.transaction_log_entries.count }.by(1)
+
+            entry = @match_score.tournament.transaction_log_entries.last
+            expect(entry.action).to eq('match_score.updated')
+            expect(entry.message).to include("changed matchscore #{@match_score.id}")
+            expect(entry.metadata['match_score_id']).to eq(@match_score.id)
+            expect(entry.metadata['changes']['points']).to eq(
+              'from' => @match_score.points,
+              'to' => valid_update[:points]
+            )
+          end
         end
 
         context 'when match_score update fails' do
diff --git a/spec/controllers/matches_controller_spec.rb b/spec/controllers/matches_controller_spec.rb
index ed1a5a0..60c92a2 100644
--- a/spec/controllers/matches_controller_spec.rb
+++ b/spec/controllers/matches_controller_spec.rb
@@ -101,6 +101,8 @@ RSpec.describe MatchesController, type: :controller do
       end
 
       it 'creates a decider match at the end of the group' do
+        log_count = group_stage_tournament.transaction_log_entries.count
+
         expect do
           post :create, params: { group_id: group.to_param, team_ids: team_ids }
         end.to change { group.matches.reload.count }.by(1)
@@ -110,6 +112,8 @@ RSpec.describe MatchesController, type: :controller do
         expect(created_match.decider?).to eq(true)
         expect(created_match.state).to eq('not_started')
         expect(created_match.teams.map(&:id)).to match_array(team_ids)
+        expect(group_stage_tournament.transaction_log_entries.count).to eq(log_count + 1)
+        expect(group_stage_tournament.transaction_log_entries.last.action).to eq('decider_match.created')
       end
     end
 
@@ -233,6 +237,21 @@ RSpec.describe MatchesController, type: :controller do
             expect(body[:state]).to eq(valid_update[:state])
           end
 
+          it 'writes a transaction log entry for the state change' do
+            expect(TournamentLiveBroadcast).to receive(:call).with(@match.tournament.id)
+
+            expect do
+              put :update, params: { id: @match.to_param }.merge(valid_update)
+            end.to change { @match.tournament.transaction_log_entries.count }.by(1)
+
+            entry = @match.tournament.transaction_log_entries.last
+            expect(entry.action).to eq('match.state_changed')
+            expect(entry.sequence).to eq(1)
+            expect(entry.message).to include("changed match #{@match.id}")
+            expect(entry.message).to include('running')
+            expect(entry.metadata['new_state']).to eq('in_progress')
+          end
+
           it 'stops an in-progress match and resets its scores' do
             @match.update!(state: :in_progress)
             @match.match_scores.each do |match_score|
diff --git a/spec/controllers/stages_controller_spec.rb b/spec/controllers/stages_controller_spec.rb
index bd7ba8c..d447eec 100644
--- a/spec/controllers/stages_controller_spec.rb
+++ b/spec/controllers/stages_controller_spec.rb
@@ -97,6 +97,13 @@ RSpec.describe StagesController, type: :controller do
           tournament = running_group_stage.tournament.reload
           expect(tournament.team_action_lists).to be_empty
         end
+
+        it 'writes a transaction log entry for starting playoffs' do
+          entry = running_group_stage.tournament.transaction_log_entries.last
+          expect(entry.action).to eq('playoffs.started')
+          expect(entry.message).to include('started playoffs')
+          expect(entry.metadata['advancing_team_ids']).to match_array(GroupStageService.get_advancing_teams(running_group_stage).map(&:id))
+        end
       end
 
       context 'as another user' do
@@ -206,6 +213,13 @@ RSpec.describe StagesController, type: :controller do
         expect(body[:blocking_ties].size).to eq(1)
         expect(body[:blocking_ties].first[:team_names]).to eq(%w[A B])
       end
+
+      it 'writes a failed playoff start transaction log entry' do
+        entry = group_stage.tournament.transaction_log_entries.last
+        expect(entry.action).to eq('playoffs.start_failed')
+        expect(entry.message).to include('failed because decider missing')
+        expect(entry.metadata['reason']).to eq('decider_missing')
+      end
     end
 
     context 'when tied advancing teams still need a decider to fix playoff seeding' do
diff --git a/spec/controllers/tournament_transaction_log_entries_controller_spec.rb b/spec/controllers/tournament_transaction_log_entries_controller_spec.rb
new file mode 100644
index 0000000..fe07049
--- /dev/null
+++ b/spec/controllers/tournament_transaction_log_entries_controller_spec.rb
@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe TournamentTransactionLogEntriesController, type: :controller do
+  let(:tournament) { create(:tournament) }
+  let(:owner) { tournament.owner }
+
+  before do
+    TournamentTransactionLogger.record!(
+      tournament: tournament,
+      user: owner,
+      action: 'test.entry',
+      message: 'owner changed match 1',
+      metadata: { match_id: 1 }
+    )
+  end
+
+  describe 'GET #index' do
+    context 'as owner' do
+      before do
+        apply_authentication_headers_for owner
+      end
+
+      it 'returns ordered transaction log entries' do
+        get :index, params: { id: tournament.to_param }
+
+        expect(response).to be_successful
+        body = deserialize_response(response)
+        expect(body.size).to eq(1)
+        expect(body.first[:sequence]).to eq(1)
+        expect(body.first[:action]).to eq('test.entry')
+        expect(body.first[:message]).to eq('owner changed match 1')
+        expect(body.first[:metadata]).to eq(match_id: 1)
+      end
+    end
+
+    context 'as another user' do
+      before do
+        apply_authentication_headers_for create(:user)
+      end
+
+      it 'returns forbidden' do
+        get :index, params: { id: tournament.to_param }
+
+        expect(response).to have_http_status(:forbidden)
+      end
+    end
+  end
+end
diff --git a/spec/e2e/http/tournament_lifecycle_spec.rb b/spec/e2e/http/tournament_lifecycle_spec.rb
index 36f8ee2..437c063 100644
--- a/spec/e2e/http/tournament_lifecycle_spec.rb
+++ b/spec/e2e/http/tournament_lifecycle_spec.rb
@@ -129,4 +129,21 @@ RSpec.describe 'Tournament lifecycle HTTP E2E' do
       expect(final_stage[:match_states]).to eq(team_count > 3 ? %w[finished finished] : ['finished'])
     end
   end
+
+  it 'records ordered owner-readable transaction history over HTTP' do
+    result = runner.run_transaction_log_history
+    log = result.dig(:checkpoints, :transaction_log)
+
+    expect(log.fetch(:sequences)).to eq((1..log.fetch(:sequences).count).to_a)
+    expect(log.fetch(:actions)).to include(
+      'tournament.created',
+      'match.state_changed',
+      'match_score.updated',
+      'playoffs.start_failed',
+      'decider_match.created',
+      'playoffs.started'
+    )
+    expect(log.fetch(:messages).join("\n")).to include('failed because decider missing')
+    expect(log.fetch(:messages).join("\n")).to include('started playoffs, advancing teams:')
+  end
 end
diff --git a/spec/routing/tournaments_routing_spec.rb b/spec/routing/tournaments_routing_spec.rb
index 3cb398a..0c185eb 100644
--- a/spec/routing/tournaments_routing_spec.rb
+++ b/spec/routing/tournaments_routing_spec.rb
@@ -41,5 +41,9 @@ RSpec.describe TournamentsController, type: :routing do
     it 'routes to #test_sync' do
       expect(post: '/tournaments/1/test_sync').to route_to('tournaments#test_sync', id: '1')
     end
+
+    it 'routes to transaction log entries' do
+      expect(get: '/tournaments/1/transaction_log').to route_to('tournament_transaction_log_entries#index', id: '1')
+    end
   end
 end
diff --git a/spec/services/tournament_transaction_logger_spec.rb b/spec/services/tournament_transaction_logger_spec.rb
new file mode 100644
index 0000000..78e2851
--- /dev/null
+++ b/spec/services/tournament_transaction_logger_spec.rb
@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe TournamentTransactionLogger do
+  describe '.record!' do
+    it 'assigns per-tournament sequences in order' do
+      tournament = create(:tournament)
+      user = tournament.owner
+
+      first = described_class.record!(
+        tournament: tournament,
+        user: user,
+        action: 'test.first',
+        message: 'first',
+        metadata: { number: 1 }
+      )
+      second = described_class.record!(
+        tournament: tournament,
+        user: user,
+        action: 'test.second',
+        message: 'second',
+        metadata: { number: 2 }
+      )
+
+      expect(first.sequence).to eq(1)
+      expect(second.sequence).to eq(2)
+      expect(tournament.transaction_log_entries.order(:sequence).pluck(:message)).to eq(%w[first second])
+    end
+
+    it 'does not clear pending after_commit change tracking on the caller tournament' do
+      tournament = create(:tournament)
+      user = tournament.owner
+      old_timer = { timestamp: nil, timer_mode: nil, timer_reason: nil, timer_reason_text: nil }
+
+      Tournament.transaction do
+        tournament.update!(
+          timestamp: 2.minutes.from_now,
+          timer_mode: 'countdown',
+          timer_reason: 'tournament_start'
+        )
+        new_timer = { timestamp: tournament.timestamp.iso8601, timer_mode: 'countdown',
+                      timer_reason: 'tournament_start', timer_reason_text: nil }
+
+        expect(tournament.saved_change_to_timestamp?).to be(true)
+
+        described_class.record_timer_changed!(
+          tournament: tournament,
+          user: user,
+          old_timer: old_timer,
+          new_timer: new_timer
+        )
+
+        expect(tournament.saved_change_to_timestamp?).to be(true)
+      end
+    end
+  end
+end