4 files changed, 67 insertions, 0 deletions
diff --git a/changelogs/unreleased/41476-enable-project-milestons-deletion-via-api.yml b/changelogs/unreleased/41476-enable-project-milestons-deletion-via-api.yml
new file mode 100644
index 00000000000..bb5c1fdf082
--- /dev/null
+++ b/changelogs/unreleased/41476-enable-project-milestons-deletion-via-api.yml
@@ -0,0 +1,5 @@
+---
+title: Enables Project Milestone Deletion via the API
+merge_request: 16478
+author: Jacopo Beschi @jacopo-beschi
+type: added
diff --git a/doc/api/milestones.md b/doc/api/milestones.md
index 84930f0bdc9..d35e940d7b1 100644
--- a/doc/api/milestones.md
+++ b/doc/api/milestones.md
@@ -93,6 +93,19 @@ Parameters:
 - `start_date` (optional) - The start date of the milestone
 - `state_event` (optional) - The state event of the milestone (close|activate)
 
+## Delete project milestone
+
+Only for user with developer access to the project.
+
+```
+DELETE /projects/:id/milestones/:milestone_id
+```
+
+Parameters:
+
+- `id` (required) - The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
+- `milestone_id` (required) - The ID of the project's milestone
+
 ## Get all issues assigned to a single milestone
 
 Gets all issues assigned to a single project milestone.
diff --git a/lib/api/project_milestones.rb b/lib/api/project_milestones.rb
index 0cb209a02d0..306dc0e63d7 100644
--- a/lib/api/project_milestones.rb
+++ b/lib/api/project_milestones.rb
@@ -60,6 +60,15 @@ module API
         update_milestone_for(user_project)
       end
 
+      desc 'Remove a project milestone'
+      delete ":id/milestones/:milestone_id" do
+        authorize! :admin_milestone, user_project
+
+        user_project.milestones.find(params[:milestone_id]).destroy
+
+        status(204)
+      end
+
       desc 'Get all issues for a single project milestone' do
         success Entities::IssueBasic
       end
diff --git a/spec/requests/api/project_milestones_spec.rb b/spec/requests/api/project_milestones_spec.rb
index 08ea7314bb3..6c05c166bd6 100644
--- a/spec/requests/api/project_milestones_spec.rb
+++ b/spec/requests/api/project_milestones_spec.rb
@@ -14,6 +14,46 @@ describe API::ProjectMilestones do
     let(:route) { "/projects/#{project.id}/milestones" }
   end
 
+  describe 'DELETE /projects/:id/milestones/:milestone_id' do
+    let(:guest) { create(:user) }
+    let(:reporter) { create(:user) }
+
+    before do
+      project.add_reporter(reporter)
+    end
+
+    it 'returns 404 response when the project does not exists' do
+      delete api("/projects/999/milestones/#{milestone.id}", user)
+
+      expect(response).to have_gitlab_http_status(404)
+    end
+
+    it 'returns 404 response when the milestone does not exists' do
+      delete api("/projects/#{project.id}/milestones/999", user)
+
+      expect(response).to have_gitlab_http_status(404)
+    end
+
+    it "returns 404 from guest user deleting a milestone" do
+      delete api("/projects/#{project.id}/milestones/#{milestone.id}", guest)
+
+      expect(response).to have_gitlab_http_status(404)
+    end
+
+    it "rejects a member with reporter access from deleting a milestone" do
+      delete api("/projects/#{project.id}/milestones/#{milestone.id}", reporter)
+
+      expect(response).to have_gitlab_http_status(403)
+    end
+
+    it 'deletes the milestone when the user has developer access to the project' do
+      delete api("/projects/#{project.id}/milestones/#{milestone.id}", user)
+
+      expect(project.milestones.find_by_id(milestone.id)).to be_nil
+      expect(response).to have_gitlab_http_status(204)
+    end
+  end
+
   describe 'PUT /projects/:id/milestones/:milestone_id to test observer on close' do
     it 'creates an activity event when an milestone is closed' do
       expect(Event).to receive(:create!)