diff --git a/app/controllers/admin/topics_controller.rb b/app/controllers/admin/topics_controller.rb
index b451928e5919a5709a14cb5df796a099ff6ba054..69bcfdf4791f96a5ba46bdabf5eed97651cdcedc 100644
--- a/app/controllers/admin/topics_controller.rb
+++ b/app/controllers/admin/topics_controller.rb
@@ -45,6 +45,22 @@ def destroy
                 notice: _('Topic %{topic_name} was successfully removed.') % { topic_name: @topic.title_or_name }
   end
 
+  def merge
+    source_topic = Projects::Topic.find(merge_params[:source_topic_id])
+    target_topic = Projects::Topic.find(merge_params[:target_topic_id])
+
+    begin
+      ::Topics::MergeService.new(source_topic, target_topic).execute
+    rescue ArgumentError => e
+      return render status: :bad_request, json: { type: :alert, message: e.message }
+    end
+
+    message = _('Topic %{source_topic} was successfully merged into topic %{target_topic}.')
+    redirect_to admin_topics_path,
+                status: :found,
+                notice: message % { source_topic: source_topic.name, target_topic: target_topic.name }
+  end
+
   private
 
   def topic
@@ -63,4 +79,8 @@ def allowed_topic_params
       :title
     ]
   end
+
+  def merge_params
+    params.permit([:source_topic_id, :target_topic_id])
+  end
 end
diff --git a/app/services/topics/merge_service.rb b/app/services/topics/merge_service.rb
new file mode 100644
index 0000000000000000000000000000000000000000..0d256579fe04d7e2396c890e74cf155eed90b857
--- /dev/null
+++ b/app/services/topics/merge_service.rb
@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+module Topics
+  class MergeService
+    attr_accessor :source_topic, :target_topic
+
+    def initialize(source_topic, target_topic)
+      @source_topic = source_topic
+      @target_topic = target_topic
+    end
+
+    def execute
+      validate_parameters!
+
+      ::Projects::ProjectTopic.transaction do
+        move_project_topics
+        refresh_target_topic_counters
+        delete_source_topic
+      end
+    end
+
+    private
+
+    def validate_parameters!
+      raise ArgumentError, 'The source topic is not a topic.' unless source_topic.is_a?(Projects::Topic)
+      raise ArgumentError, 'The target topic is not a topic.' unless target_topic.is_a?(Projects::Topic)
+      raise ArgumentError, 'The source topic and the target topic are identical.' if source_topic == target_topic
+    end
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def move_project_topics
+      project_ids_for_projects_currently_using_source_and_target = ::Projects::ProjectTopic
+        .where(topic_id: target_topic).select(:project_id)
+      # Only update for projects that exclusively use the source topic
+      ::Projects::ProjectTopic.where(topic_id: source_topic.id)
+        .where.not(project_id: project_ids_for_projects_currently_using_source_and_target)
+        .update_all(topic_id: target_topic.id)
+
+      # Delete source topic for projects that were using source and target
+      ::Projects::ProjectTopic.where(topic_id: source_topic.id).delete_all
+    end
+
+    def refresh_target_topic_counters
+      target_topic.update!(
+        total_projects_count: total_projects_count(target_topic.id),
+        non_private_projects_count: non_private_projects_count(target_topic.id)
+      )
+    end
+
+    def delete_source_topic
+      source_topic.destroy!
+    end
+
+    def total_projects_count(topic_id)
+      ::Projects::ProjectTopic.where(topic_id: topic_id).count
+    end
+
+    def non_private_projects_count(topic_id)
+      ::Projects::ProjectTopic.joins(:project).where(topic_id: topic_id).where('projects.visibility_level in (10, 20)')
+        .count
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+  end
+end
diff --git a/config/routes/admin.rb b/config/routes/admin.rb
index bbf00cd0b002bc0198f287fc15a9998357a605d2..2f4e286f5eb24f5f4d7701e4132c18b8f380e9b2 100644
--- a/config/routes/admin.rb
+++ b/config/routes/admin.rb
@@ -65,6 +65,7 @@
     resource :avatar, controller: 'topics/avatars', only: [:destroy]
     collection do
       post :preview_markdown
+      post :merge
     end
   end
 
diff --git a/locale/gitlab.pot b/locale/gitlab.pot
index 2c2ce654412c36305359f749dc4174f4f536c7dc..555eb473c1dfd3025d105ee46c7ac1c218a848ef 100644
--- a/locale/gitlab.pot
+++ b/locale/gitlab.pot
@@ -40998,6 +40998,9 @@ msgstr ""
 msgid "TopNav|Go back"
 msgstr ""
 
+msgid "Topic %{source_topic} was successfully merged into topic %{target_topic}."
+msgstr ""
+
 msgid "Topic %{topic_name} was successfully created."
 msgstr ""
 
diff --git a/spec/controllers/admin/topics_controller_spec.rb b/spec/controllers/admin/topics_controller_spec.rb
index ee36d5f1def6694b12faf06b58cf114a314c8a23..87093e0263bbbfafe870f93d9884e3b38e32e6e1 100644
--- a/spec/controllers/admin/topics_controller_spec.rb
+++ b/spec/controllers/admin/topics_controller_spec.rb
@@ -173,4 +173,44 @@
       end
     end
   end
+
+  describe 'POST #merge' do
+    let_it_be(:source_topic) { create(:topic, name: 'source_topic') }
+    let_it_be(:project) { create(:project, topic_list: source_topic.name ) }
+
+    it 'merges source topic into target topic' do
+      post :merge, params: { source_topic_id: source_topic.id, target_topic_id: topic.id }
+
+      expect(response).to redirect_to(admin_topics_path)
+      expect(topic.projects).to contain_exactly(project)
+      expect { source_topic.reload }.to raise_error(ActiveRecord::RecordNotFound)
+    end
+
+    it 'renders a 404 error for non-existing id' do
+      post :merge, params: { source_topic_id: non_existing_record_id, target_topic_id: topic.id }
+
+      expect(response).to have_gitlab_http_status(:not_found)
+      expect { topic.reload }.not_to raise_error
+    end
+
+    it 'renders a 400 error for identical topic ids' do
+      post :merge, params: { source_topic_id: topic, target_topic_id: topic.id }
+
+      expect(response).to have_gitlab_http_status(:bad_request)
+      expect { topic.reload }.not_to raise_error
+    end
+
+    context 'as a normal user' do
+      before do
+        sign_in(user)
+      end
+
+      it 'renders a 404 error' do
+        post :merge, params: { source_topic_id: source_topic.id, target_topic_id: topic.id }
+
+        expect(response).to have_gitlab_http_status(:not_found)
+        expect { source_topic.reload }.not_to raise_error
+      end
+    end
+  end
 end
diff --git a/spec/services/topics/merge_service_spec.rb b/spec/services/topics/merge_service_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..971917eb8e9bd4a5ab2bb543eaf519196718697b
--- /dev/null
+++ b/spec/services/topics/merge_service_spec.rb
@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Topics::MergeService do
+  let_it_be(:source_topic) { create(:topic, name: 'source_topic') }
+  let_it_be(:target_topic) { create(:topic, name: 'target_topic') }
+  let_it_be(:project_1) { create(:project, :public, topic_list: source_topic.name ) }
+  let_it_be(:project_2) { create(:project, :private, topic_list: source_topic.name ) }
+  let_it_be(:project_3) { create(:project, :public, topic_list: target_topic.name ) }
+  let_it_be(:project_4) { create(:project, :public, topic_list: [source_topic.name, target_topic.name] ) }
+
+  subject { described_class.new(source_topic, target_topic).execute }
+
+  describe '#execute' do
+    it 'merges source topic into target topic' do
+      subject
+
+      expect(target_topic.projects).to contain_exactly(project_1, project_2, project_3, project_4)
+      expect { source_topic.reload }.to raise_error(ActiveRecord::RecordNotFound)
+    end
+
+    it 'refreshes counters of target topic' do
+      expect { subject }
+        .to change { target_topic.reload.total_projects_count }.by(2)
+        .and change { target_topic.reload.non_private_projects_count }.by(1)
+    end
+
+    context 'when source topic fails to delete' do
+      it 'reverts previous changes' do
+        allow(source_topic.reload).to receive(:destroy!).and_raise(ActiveRecord::RecordNotDestroyed)
+
+        expect { subject }.to raise_error(ActiveRecord::RecordNotDestroyed)
+
+        expect(source_topic.projects).to contain_exactly(project_1, project_2, project_4)
+        expect(target_topic.projects).to contain_exactly(project_3, project_4)
+      end
+    end
+
+    context 'for parameter validation' do
+      using RSpec::Parameterized::TableSyntax
+
+      subject { described_class.new(source_topic_parameter, target_topic_parameter).execute }
+
+      where(:source_topic_parameter, :target_topic_parameter, :expected_message) do
+        nil                | ref(:target_topic) | 'The source topic is not a topic.'
+        ref(:source_topic) | nil                | 'The target topic is not a topic.'
+        ref(:target_topic) | ref(:target_topic) | 'The source topic and the target topic are identical.' # rubocop:disable Lint/BinaryOperatorWithIdenticalOperands
+      end
+
+      with_them do
+        it 'raises correct error' do
+          expect { subject }.to raise_error(ArgumentError) do |error|
+            expect(error.message).to eq(expected_message)
+          end
+        end
+      end
+    end
+  end
+end