diff --git a/changelogs/unreleased/fix-40798-namespace-forking.yml b/changelogs/unreleased/fix-40798-namespace-forking.yml
new file mode 100644
index 00000000000..095235725f8
--- /dev/null
+++ b/changelogs/unreleased/fix-40798-namespace-forking.yml
@@ -0,0 +1,5 @@
+---
+title: Fix forking to subgroup via API when namespace is given by name
+merge_request: 17815
+author: Jan Beckmann
+type: fixed
diff --git a/lib/api/projects.rb b/lib/api/projects.rb
index b552b0e0c5d..467bc78dad8 100644
--- a/lib/api/projects.rb
+++ b/lib/api/projects.rb
@@ -228,11 +228,7 @@ module API
         namespace_id = fork_params[:namespace]
 
         if namespace_id.present?
-          fork_params[:namespace] = if namespace_id =~ /^\d+$/
-                                      Namespace.find_by(id: namespace_id)
-                                    else
-                                      Namespace.find_by_path_or_name(namespace_id)
-                                    end
+          fork_params[:namespace] = find_namespace(namespace_id)
 
           unless fork_params[:namespace] && can?(current_user, :create_projects, fork_params[:namespace])
             not_found!('Target Namespace')
diff --git a/lib/api/v3/projects.rb b/lib/api/v3/projects.rb
index 7d8b1f369fe..a2df969d819 100644
--- a/lib/api/v3/projects.rb
+++ b/lib/api/v3/projects.rb
@@ -268,11 +268,7 @@ module API
           namespace_id = fork_params[:namespace]
 
           if namespace_id.present?
-            fork_params[:namespace] = if namespace_id =~ /^\d+$/
-                                        Namespace.find_by(id: namespace_id)
-                                      else
-                                        Namespace.find_by_path_or_name(namespace_id)
-                                      end
+            fork_params[:namespace] = find_namespace(namespace_id)
 
             unless fork_params[:namespace] && can?(current_user, :create_projects, fork_params[:namespace])
               not_found!('Target Namespace')
diff --git a/spec/requests/api/projects_spec.rb b/spec/requests/api/projects_spec.rb
index cee93f6ed14..d73a42f48ad 100644
--- a/spec/requests/api/projects_spec.rb
+++ b/spec/requests/api/projects_spec.rb
@@ -1718,6 +1718,12 @@ describe API::Projects do
       group
     end
 
+    let(:group3) do
+      group = create(:group, name: 'group3_name', parent: group2)
+      group.add_owner(user2)
+      group
+    end
+
     before do
       project.add_reporter(user2)
     end
@@ -1813,6 +1819,15 @@ describe API::Projects do
         expect(json_response['namespace']['name']).to eq(group2.name)
       end
 
+      it 'forks to owned subgroup' do
+        full_path = "#{group2.path}/#{group3.path}"
+        post api("/projects/#{project.id}/fork", user2), namespace: full_path
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['namespace']['name']).to eq(group3.name)
+        expect(json_response['namespace']['full_path']).to eq(full_path)
+      end
+
       it 'fails to fork to not owned group' do
         post api("/projects/#{project.id}/fork", user2), namespace: group.name