diff --git a/Gemfile b/Gemfile
index 1264d75eac6..b1750afec91 100644
--- a/Gemfile
+++ b/Gemfile
@@ -429,7 +429,7 @@ group :ed25519 do
 end
 
 # Gitaly GRPC client
-gem 'gitaly-proto', '~> 1.32.0', require: 'gitaly'
+gem 'gitaly-proto', '~> 1.36.0', require: 'gitaly'
 
 gem 'grpc', '~> 1.19.0'
 
diff --git a/Gemfile.lock b/Gemfile.lock
index 5b648d43137..5099167e03f 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -303,7 +303,7 @@ GEM
       gettext_i18n_rails (>= 0.7.1)
       po_to_json (>= 1.0.0)
       rails (>= 3.2.0)
-    gitaly-proto (1.32.0)
+    gitaly-proto (1.36.0)
       grpc (~> 1.0)
     github-markup (1.7.0)
     gitlab-labkit (0.3.0)
@@ -1092,7 +1092,7 @@ DEPENDENCIES
   gettext (~> 3.2.2)
   gettext_i18n_rails (~> 1.8.0)
   gettext_i18n_rails_js (~> 1.3)
-  gitaly-proto (~> 1.32.0)
+  gitaly-proto (~> 1.36.0)
   github-markup (~> 1.7.0)
   gitlab-labkit (~> 0.3.0)
   gitlab-markup (~> 1.7.0)
diff --git a/app/models/repository.rb b/app/models/repository.rb
index d087a5a7bbd..a408db7ebbe 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -839,10 +839,10 @@ class Repository
     end
   end
 
-  def merge_to_ref(user, source_sha, merge_request, target_ref, message)
+  def merge_to_ref(user, source_sha, merge_request, target_ref, message, first_parent_ref)
     branch = merge_request.target_branch
 
-    raw.merge_to_ref(user, source_sha, branch, target_ref, message)
+    raw.merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
   end
 
   def ff_merge(user, source, target_branch, merge_request: nil)
diff --git a/app/services/merge_requests/merge_to_ref_service.rb b/app/services/merge_requests/merge_to_ref_service.rb
index efe4dcd6255..0ea50a5dbf5 100644
--- a/app/services/merge_requests/merge_to_ref_service.rb
+++ b/app/services/merge_requests/merge_to_ref_service.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module MergeRequests
-  # Performs the merge between source SHA and the target branch. Instead
+  # Performs the merge between source SHA and the target branch or the specified first parent ref. Instead
   # of writing the result to the MR target branch, it targets the `target_ref`.
   #
   # Ideally this should leave the `target_ref` state with the same state the
@@ -56,12 +56,22 @@ module MergeRequests
       raise_error(error) if error
     end
 
+    ##
+    # The parameter `target_ref` is where the merge result will be written.
+    # Default is the merge ref i.e. `refs/merge-requests/:iid/merge`.
     def target_ref
-      merge_request.merge_ref_path
+      params[:target_ref] || merge_request.merge_ref_path
+    end
+
+    ##
+    # The parameter `first_parent_ref` is the main line of the merge commit.
+    # Default is the target branch ref of the merge request.
+    def first_parent_ref
+      params[:first_parent_ref] || merge_request.target_branch_ref
     end
 
     def commit
-      repository.merge_to_ref(current_user, source, merge_request, target_ref, commit_message)
+      repository.merge_to_ref(current_user, source, merge_request, target_ref, commit_message, first_parent_ref)
     rescue Gitlab::Git::PreReceiveError => error
       raise MergeError, error.message
     end
diff --git a/changelogs/unreleased/create-merge-train-ref-ce.yml b/changelogs/unreleased/create-merge-train-ref-ce.yml
new file mode 100644
index 00000000000..b0b95275f58
--- /dev/null
+++ b/changelogs/unreleased/create-merge-train-ref-ce.yml
@@ -0,0 +1,5 @@
+---
+title: Extend `MergeToRefService` to create merge ref from an arbitrary ref
+merge_request: 30361
+author:
+type: added
diff --git a/lib/gitlab/git/repository.rb b/lib/gitlab/git/repository.rb
index 19b6aab1c4f..060a29be782 100644
--- a/lib/gitlab/git/repository.rb
+++ b/lib/gitlab/git/repository.rb
@@ -536,9 +536,9 @@ module Gitlab
         tags.find { |tag| tag.name == name }
       end
 
-      def merge_to_ref(user, source_sha, branch, target_ref, message)
+      def merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
         wrapped_gitaly_errors do
-          gitaly_operation_client.user_merge_to_ref(user, source_sha, branch, target_ref, message)
+          gitaly_operation_client.user_merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
         end
       end
 
diff --git a/lib/gitlab/gitaly_client/operation_service.rb b/lib/gitlab/gitaly_client/operation_service.rb
index b42e6cbad8d..783c2ff0915 100644
--- a/lib/gitlab/gitaly_client/operation_service.rb
+++ b/lib/gitlab/gitaly_client/operation_service.rb
@@ -100,14 +100,15 @@ module Gitlab
         end
       end
 
-      def user_merge_to_ref(user, source_sha, branch, target_ref, message)
+      def user_merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
         request = Gitaly::UserMergeToRefRequest.new(
           repository: @gitaly_repo,
           source_sha: source_sha,
           branch: encode_binary(branch),
           target_ref: encode_binary(target_ref),
           user: Gitlab::Git::User.from_gitlab(user).to_gitaly,
-          message: encode_binary(message)
+          message: encode_binary(message),
+          first_parent_ref: encode_binary(first_parent_ref)
         )
 
         response = GitalyClient.call(@repository.storage, :operation_service, :user_merge_to_ref, request)
diff --git a/spec/lib/gitlab/git/repository_spec.rb b/spec/lib/gitlab/git/repository_spec.rb
index cceeae8afe6..a28b95e5bff 100644
--- a/spec/lib/gitlab/git/repository_spec.rb
+++ b/spec/lib/gitlab/git/repository_spec.rb
@@ -1694,14 +1694,15 @@ describe Gitlab::Git::Repository, :seed_helper do
     let(:branch_head) { '6d394385cf567f80a8fd85055db1ab4c5295806f' }
     let(:left_sha) { 'cfe32cf61b73a0d5e9f13e774abde7ff789b1660' }
     let(:right_branch) { 'test-master' }
+    let(:first_parent_ref) { 'refs/heads/test-master' }
     let(:target_ref) { 'refs/merge-requests/999/merge' }
 
     before do
-      repository.create_branch(right_branch, branch_head) unless repository.branch_exists?(right_branch)
+      repository.create_branch(right_branch, branch_head) unless repository.ref_exists?(first_parent_ref)
     end
 
     def merge_to_ref
-      repository.merge_to_ref(user, left_sha, right_branch, target_ref, 'Merge message')
+      repository.merge_to_ref(user, left_sha, right_branch, target_ref, 'Merge message', first_parent_ref)
     end
 
     it 'generates a commit in the target_ref' do
@@ -1716,7 +1717,7 @@ describe Gitlab::Git::Repository, :seed_helper do
     end
 
     it 'does not change the right branch HEAD' do
-      expect { merge_to_ref }.not_to change { repository.find_branch(right_branch).target }
+      expect { merge_to_ref }.not_to change { repository.commit(first_parent_ref).sha }
     end
   end
 
diff --git a/spec/lib/gitlab/gitaly_client/operation_service_spec.rb b/spec/lib/gitlab/gitaly_client/operation_service_spec.rb
index 18663a72fcd..f38b8d31237 100644
--- a/spec/lib/gitlab/gitaly_client/operation_service_spec.rb
+++ b/spec/lib/gitlab/gitaly_client/operation_service_spec.rb
@@ -79,13 +79,13 @@ describe Gitlab::GitalyClient::OperationService do
     end
 
     describe '#user_merge_to_ref' do
-      let(:branch) { 'my-branch' }
+      let(:first_parent_ref) { 'refs/heads/my-branch' }
       let(:source_sha) { 'cfe32cf61b73a0d5e9f13e774abde7ff789b1660' }
       let(:ref) { 'refs/merge-requests/x/merge' }
       let(:message) { 'validación' }
       let(:response) { Gitaly::UserMergeToRefResponse.new(commit_id: 'new-commit-id') }
 
-      subject { client.user_merge_to_ref(user, source_sha, branch, ref, message) }
+      subject { client.user_merge_to_ref(user, source_sha, nil, ref, message, first_parent_ref) }
 
       it 'sends a user_merge_to_ref message' do
         expect_any_instance_of(Gitaly::OperationService::Stub)
diff --git a/spec/models/repository_spec.rb b/spec/models/repository_spec.rb
index 13da7bd7407..3d967aa4ab8 100644
--- a/spec/models/repository_spec.rb
+++ b/spec/models/repository_spec.rb
@@ -1420,12 +1420,13 @@ describe Repository do
                              source_project: project)
     end
 
-    it 'writes merge of source and target to MR merge_ref_path' do
+    it 'writes merge of source SHA and first parent ref to MR merge_ref_path' do
       merge_commit_id = repository.merge_to_ref(user,
                                                 merge_request.diff_head_sha,
                                                 merge_request,
                                                 merge_request.merge_ref_path,
-                                                'Custom message')
+                                                'Custom message',
+                                                merge_request.target_branch_ref)
 
       merge_commit = repository.commit(merge_commit_id)
 
diff --git a/spec/services/merge_requests/merge_to_ref_service_spec.rb b/spec/services/merge_requests/merge_to_ref_service_spec.rb
index 61f99f82a76..e2f201677fa 100644
--- a/spec/services/merge_requests/merge_to_ref_service_spec.rb
+++ b/spec/services/merge_requests/merge_to_ref_service_spec.rb
@@ -22,7 +22,6 @@ describe MergeRequests::MergeToRefService do
   shared_examples_for 'successfully merges to ref with merge method' do
     it 'writes commit to merge ref' do
       repository = project.repository
-      target_ref = merge_request.merge_ref_path
 
       expect(repository.ref_exists?(target_ref)).to be(false)
 
@@ -33,7 +32,7 @@ describe MergeRequests::MergeToRefService do
       expect(result[:status]).to eq(:success)
       expect(result[:commit_id]).to be_present
       expect(result[:source_id]).to eq(merge_request.source_branch_sha)
-      expect(result[:target_id]).to eq(merge_request.target_branch_sha)
+      expect(result[:target_id]).to eq(repository.commit(first_parent_ref).sha)
       expect(repository.ref_exists?(target_ref)).to be(true)
       expect(ref_head.id).to eq(result[:commit_id])
     end
@@ -74,17 +73,22 @@ describe MergeRequests::MergeToRefService do
 
   describe '#execute' do
     let(:service) do
-      described_class.new(project, user, commit_message: 'Awesome message',
-                                         should_remove_source_branch: true)
+      described_class.new(project, user, **params)
     end
 
+    let(:params) { { commit_message: 'Awesome message', should_remove_source_branch: true } }
+
     def process_merge_to_ref
       perform_enqueued_jobs do
         service.execute(merge_request)
       end
     end
 
-    it_behaves_like 'successfully merges to ref with merge method'
+    it_behaves_like 'successfully merges to ref with merge method' do
+      let(:first_parent_ref) { 'refs/heads/master' }
+      let(:target_ref) { merge_request.merge_ref_path }
+    end
+
     it_behaves_like 'successfully evaluates pre-condition checks'
 
     context 'commit history comparison with regular MergeService' do
@@ -129,14 +133,22 @@ describe MergeRequests::MergeToRefService do
       context 'when semi-linear merge method' do
         let(:merge_method) { :rebase_merge }
 
-        it_behaves_like 'successfully merges to ref with merge method'
+        it_behaves_like 'successfully merges to ref with merge method' do
+          let(:first_parent_ref) { 'refs/heads/master' }
+          let(:target_ref) { merge_request.merge_ref_path }
+        end
+
         it_behaves_like 'successfully evaluates pre-condition checks'
       end
 
       context 'when fast-forward merge method' do
         let(:merge_method) { :ff }
 
-        it_behaves_like 'successfully merges to ref with merge method'
+        it_behaves_like 'successfully merges to ref with merge method' do
+          let(:first_parent_ref) { 'refs/heads/master' }
+          let(:target_ref) { merge_request.merge_ref_path }
+        end
+
         it_behaves_like 'successfully evaluates pre-condition checks'
       end
 
@@ -178,5 +190,34 @@ describe MergeRequests::MergeToRefService do
 
       it { expect(todo).not_to be_done }
     end
+
+    describe 'cascading merge refs' do
+      set(:project) { create(:project, :repository) }
+      let(:params) { { commit_message: 'Cascading merge', first_parent_ref: first_parent_ref, target_ref: target_ref } }
+
+      context 'when first merge happens' do
+        let(:merge_request) do
+          create(:merge_request, source_project: project, source_branch: 'feature',
+                                 target_project: project, target_branch: 'master')
+        end
+
+        it_behaves_like 'successfully merges to ref with merge method' do
+          let(:first_parent_ref) { 'refs/heads/master' }
+          let(:target_ref) { 'refs/merge-requests/1/train' }
+        end
+
+        context 'when second merge happens' do
+          let(:merge_request) do
+            create(:merge_request, source_project: project, source_branch: 'improve/awesome',
+                                   target_project: project, target_branch: 'master')
+          end
+
+          it_behaves_like 'successfully merges to ref with merge method' do
+            let(:first_parent_ref) { 'refs/merge-requests/1/train' }
+            let(:target_ref) { 'refs/merge-requests/2/train' }
+          end
+        end
+      end
+    end
   end
 end