diff --git a/changelogs/unreleased/55127-add-delay-after-mr-creation-for-async-tasks-to-complete.yml b/changelogs/unreleased/55127-add-delay-after-mr-creation-for-async-tasks-to-complete.yml
new file mode 100644
index 00000000000..ac3bb596842
--- /dev/null
+++ b/changelogs/unreleased/55127-add-delay-after-mr-creation-for-async-tasks-to-complete.yml
@@ -0,0 +1,5 @@
+---
+title: Wait for pipeline creation to complete before accepting a MR via API
+merge_request: 27978
+author: kerrizor
+type: fixed
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index ce85772e4ed..daa98c22e5e 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -367,6 +367,10 @@ module API
         merge_request = find_project_merge_request(params[:merge_request_iid])
         merge_when_pipeline_succeeds = to_boolean(params[:merge_when_pipeline_succeeds])
 
+        if merge_when_pipeline_succeeds || merge_request.merge_when_pipeline_succeeds
+          render_api_error!('Not allowed: pipeline does not exist', 405) unless merge_request.head_pipeline
+        end
+
         # Merge request can not be merged
         # because user dont have permissions to push into target branch
         unauthorized! unless merge_request.can_be_merged_by?(current_user)
diff --git a/spec/requests/api/merge_requests_spec.rb b/spec/requests/api/merge_requests_spec.rb
index 5c94a87529b..007f3517e64 100644
--- a/spec/requests/api/merge_requests_spec.rb
+++ b/spec/requests/api/merge_requests_spec.rb
@@ -1495,6 +1495,33 @@ describe API::MergeRequests do
       expect(json_response['merge_when_pipeline_succeeds']).to eq(true)
     end
 
+    context 'when the MR requires pipeline success' do
+      it 'returns 405 if the pipeline is missing' do
+        allow_any_instance_of(MergeRequest)
+          .to receive(:merge_when_pipeline_succeeds).and_return(true)
+        allow_any_instance_of(MergeRequest).to receive(:head_pipeline).and_return(nil)
+
+        put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user)
+
+        expect(response).to have_gitlab_http_status(405)
+        expect(json_response['message']).to eq('Not allowed: pipeline does not exist')
+      end
+    end
+
+    context 'when the request requires pipeline success' do
+      it 'returns 405 if the pipeline is missing' do
+        allow_any_instance_of(MergeRequest)
+          .to receive(:merge_when_pipeline_succeeds).and_return(true)
+        allow_any_instance_of(MergeRequest).to receive(:head_pipeline).and_return(nil)
+
+        put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user),
+              params: { merge_when_pipeline_succeeds: true }
+
+        expect(response).to have_gitlab_http_status(405)
+        expect(json_response['message']).to eq('Not allowed: pipeline does not exist')
+      end
+    end
+
     it "returns 404 for an invalid merge request IID" do
       put api("/projects/#{project.id}/merge_requests/12345/merge", user)