diff --git a/changelogs/unreleased/3874-correctly-return-json-on-delete-responses.yml b/changelogs/unreleased/3874-correctly-return-json-on-delete-responses.yml
new file mode 100644
index 00000000000..4a4932288b4
--- /dev/null
+++ b/changelogs/unreleased/3874-correctly-return-json-on-delete-responses.yml
@@ -0,0 +1,4 @@
+---
+title: Return 202 with JSON body on async removals on V4 API
+merge_request:
+author:
diff --git a/doc/api/v3_to_v4.md b/doc/api/v3_to_v4.md
index 9a48d63c117..e141723b580 100644
--- a/doc/api/v3_to_v4.md
+++ b/doc/api/v3_to_v4.md
@@ -41,3 +41,5 @@ changes are in V4:
 - Renamed `branch_name` to `branch` on DELETE `id/repository/branches/:branch` response [!8936](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8936)
 - Remove `public` param from create and edit actions of projects [!8736](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8736)
 - Notes do not return deprecated field `upvote` and `downvote` [!9384](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9384)
+- Return 202 with JSON body on async removals on V4 API (DELETE `/projects/:id/repository/merged_branches` and DELETE `/projects/:id`) [!9449](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9449)
+
diff --git a/lib/api/branches.rb b/lib/api/branches.rb
index c65de90cca2..34f136948c2 100644
--- a/lib/api/branches.rb
+++ b/lib/api/branches.rb
@@ -137,7 +137,7 @@ module API
       delete ":id/repository/merged_branches" do
         DeleteMergedBranchesService.new(user_project, current_user).async_execute
 
-        status(200)
+        accepted!
       end
     end
   end
diff --git a/lib/api/helpers.rb b/lib/api/helpers.rb
index d0efa7b993b..72d2b320077 100644
--- a/lib/api/helpers.rb
+++ b/lib/api/helpers.rb
@@ -209,6 +209,10 @@ module API
       render_api_error!('204 No Content', 204)
     end
 
+    def accepted!
+      render_api_error!('202 Accepted', 202)
+    end
+
     def render_validation_error!(model)
       if model.errors.any?
         render_api_error!(model.errors.messages || '400 Bad Request', 400)
diff --git a/lib/api/projects.rb b/lib/api/projects.rb
index e7b891bd92e..b89bddc7e29 100644
--- a/lib/api/projects.rb
+++ b/lib/api/projects.rb
@@ -282,6 +282,8 @@ module API
       delete ":id" do
         authorize! :remove_project, user_project
         ::Projects::DestroyService.new(user_project, current_user, {}).async_execute
+
+        accepted!
       end
 
       desc 'Mark this project as forked from another'
diff --git a/lib/api/v3/branches.rb b/lib/api/v3/branches.rb
index 733c6b21be5..51eb566cf7d 100644
--- a/lib/api/v3/branches.rb
+++ b/lib/api/v3/branches.rb
@@ -18,6 +18,13 @@ module API
 
           present branches, with: ::API::Entities::RepoBranch, project: user_project
         end
+
+        desc 'Delete all merged branches'
+        delete ":id/repository/merged_branches" do
+          DeleteMergedBranchesService.new(user_project, current_user).async_execute
+
+          status(200)
+        end
       end
     end
   end
diff --git a/spec/requests/api/branches_spec.rb b/spec/requests/api/branches_spec.rb
index 5571f6cc107..cacdb21c692 100644
--- a/spec/requests/api/branches_spec.rb
+++ b/spec/requests/api/branches_spec.rb
@@ -360,9 +360,11 @@ describe API::Branches, api: true  do
       allow_any_instance_of(Repository).to receive(:rm_branch).and_return(true)
     end
 
-    it 'returns 200' do
+    it 'returns 202 with json body' do
       delete api("/projects/#{project.id}/repository/merged_branches", user)
-      expect(response).to have_http_status(200)
+
+      expect(response).to have_http_status(202)
+      expect(json_response['message']).to eql('202 Accepted')
     end
 
     it 'returns a 403 error if guest' do
diff --git a/spec/requests/api/projects_spec.rb b/spec/requests/api/projects_spec.rb
index 8d139782fdf..5de4426f3bd 100644
--- a/spec/requests/api/projects_spec.rb
+++ b/spec/requests/api/projects_spec.rb
@@ -1263,7 +1263,9 @@ describe API::Projects, api: true  do
     context 'when authenticated as user' do
       it 'removes project' do
         delete api("/projects/#{project.id}", user)
-        expect(response).to have_http_status(200)
+
+        expect(response).to have_http_status(202)
+        expect(json_response['message']).to eql('202 Accepted')
       end
 
       it 'does not remove a project if not an owner' do
@@ -1287,7 +1289,9 @@ describe API::Projects, api: true  do
     context 'when authenticated as admin' do
       it 'removes any existing project' do
         delete api("/projects/#{project.id}", admin)
-        expect(response).to have_http_status(200)
+
+        expect(response).to have_http_status(202)
+        expect(json_response['message']).to eql('202 Accepted')
       end
 
       it 'does not remove a non existing project' do
diff --git a/spec/requests/api/v3/branches_spec.rb b/spec/requests/api/v3/branches_spec.rb
index 0e4c6bc3bc6..a3e1581fcc5 100644
--- a/spec/requests/api/v3/branches_spec.rb
+++ b/spec/requests/api/v3/branches_spec.rb
@@ -20,4 +20,25 @@ describe API::V3::Branches, api: true  do
       expect(branch_names).to match_array(project.repository.branch_names)
     end
   end
+
+  describe "DELETE /projects/:id/repository/merged_branches" do
+    before do
+      allow_any_instance_of(Repository).to receive(:rm_branch).and_return(true)
+    end
+
+    it 'returns 200' do
+      delete v3_api("/projects/#{project.id}/repository/merged_branches", user)
+
+      expect(response).to have_http_status(200)
+    end
+
+    it 'returns a 403 error if guest' do
+      user_b = create :user
+      create(:project_member, :guest, user: user_b, project: project)
+
+      delete v3_api("/projects/#{project.id}/repository/merged_branches", user_b)
+
+      expect(response).to have_http_status(403)
+    end
+  end
 end