diff --git a/CHANGELOG b/CHANGELOG
index 7911e8e48c6..c26ceaead07 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -46,6 +46,7 @@ v 8.5.0 (unreleased)
   - Deprecate API "merge_request/:merge_request_id/comments". Use "merge_requests/:merge_request_id/notes" instead
   - Deprecate API "merge_request/:merge_request_id/...". Use "merge_requests/:merge_request_id/..." instead
   - Prevent parse error when name of project ends with .atom and prevent path issues
+  - Discover branches for commit statuses ref-less when doing merge when succeeded
   - Mark inline difference between old and new paths when a file is renamed
   - Support Akismet spam checking for creation of issues via API (Stan Hu)
   - API: Allow to set or update a merge-request's milestone (Kirill Skachkov)
diff --git a/app/services/merge_requests/merge_when_build_succeeds_service.rb b/app/services/merge_requests/merge_when_build_succeeds_service.rb
index 5cf7404a493..531bbc9b067 100644
--- a/app/services/merge_requests/merge_when_build_succeeds_service.rb
+++ b/app/services/merge_requests/merge_when_build_succeeds_service.rb
@@ -19,8 +19,8 @@ module MergeRequests
     end
 
     # Triggers the automatic merge of merge_request once the build succeeds
-    def trigger(build)
-      merge_requests = merge_request_from(build)
+    def trigger(commit_status)
+      merge_requests = merge_request_from(commit_status)
 
       merge_requests.each do |merge_request|
         next unless merge_request.merge_when_build_succeeds?
@@ -45,9 +45,14 @@ module MergeRequests
 
     private
 
-    def merge_request_from(build)
-      merge_requests = @project.origin_merge_requests.opened.where(source_branch: build.ref).to_a
-      merge_requests += @project.fork_merge_requests.opened.where(source_branch: build.ref).to_a
+    def merge_request_from(commit_status)
+      branches = commit_status.ref
+
+      # This is for ref-less builds
+      branches ||= @project.repository.branch_names_contains(commit_status.sha)
+
+      merge_requests = @project.origin_merge_requests.opened.where(source_branch: branches).to_a
+      merge_requests += @project.fork_merge_requests.opened.where(source_branch: branches).to_a
 
       merge_requests.uniq.select(&:source_project)
     end
diff --git a/spec/services/merge_requests/merge_when_build_succeeds_service_spec.rb b/spec/services/merge_requests/merge_when_build_succeeds_service_spec.rb
index 160c7786bf2..f285517cdac 100644
--- a/spec/services/merge_requests/merge_when_build_succeeds_service_spec.rb
+++ b/spec/services/merge_requests/merge_when_build_succeeds_service_spec.rb
@@ -54,14 +54,38 @@ describe MergeRequests::MergeWhenBuildSucceedsService do
   end
 
   describe "#trigger" do
-    let(:build)     { create(:ci_build, ref: mr_merge_if_green_enabled.source_branch, status: "success") }
+    context 'build with ref' do
+      let(:build)     { create(:ci_build, ref: mr_merge_if_green_enabled.source_branch, status: "success") }
 
-    it "merges all merge requests with merge when build succeeds enabled" do
-      allow_any_instance_of(MergeRequest).to receive(:ci_commit).and_return(ci_commit)
-      allow(ci_commit).to receive(:success?).and_return(true)
+      it "merges all merge requests with merge when build succeeds enabled" do
+        allow_any_instance_of(MergeRequest).to receive(:ci_commit).and_return(ci_commit)
+        allow(ci_commit).to receive(:success?).and_return(true)
 
-      expect(MergeWorker).to receive(:perform_async)
-      service.trigger(build)
+        expect(MergeWorker).to receive(:perform_async)
+        service.trigger(build)
+      end
+    end
+
+    context 'commit status without ref' do
+      let(:commit_status) { create(:generic_commit_status, status: 'success') }
+
+      it "doesn't merge a requests for status on other branch" do
+        allow(project.repository).to receive(:branch_names_contains).with(commit_status.sha).and_return([])
+
+        expect(MergeWorker).to_not receive(:perform_async)
+        service.trigger(commit_status)
+      end
+
+      it 'discovers branches and merges all merge requests when status is success' do
+        allow(project.repository).to receive(:branch_names_contains).
+          with(commit_status.sha).and_return([mr_merge_if_green_enabled.source_branch])
+        allow(ci_commit).to receive(:success?).and_return(true)
+        allow_any_instance_of(MergeRequest).to receive(:ci_commit).and_return(ci_commit)
+        allow(ci_commit).to receive(:success?).and_return(true)
+
+        expect(MergeWorker).to receive(:perform_async)
+        service.trigger(commit_status)
+      end
     end
 
     context 'properly handles multiple stages' do