diff --git a/changelogs/unreleased/32821-better-error-message-add-invalid-user-to-project.yml b/changelogs/unreleased/32821-better-error-message-add-invalid-user-to-project.yml
new file mode 100644
index 00000000000..587d7209c2f
--- /dev/null
+++ b/changelogs/unreleased/32821-better-error-message-add-invalid-user-to-project.yml
@@ -0,0 +1,5 @@
+---
+title: Improve error message when adding invalid user to a project
+merge_request: 20885
+author: Jacopo Beschi @jacopo-beschi
+type: added
diff --git a/lib/api/members.rb b/lib/api/members.rb
index 3d2220fed96..d23dd834c69 100644
--- a/lib/api/members.rb
+++ b/lib/api/members.rb
@@ -75,7 +75,10 @@ module API
           member = source.members.find_by(user_id: params[:user_id])
           conflict!('Member already exists') if member
 
-          member = source.add_user(params[:user_id], params[:access_level], current_user: current_user, expires_at: params[:expires_at])
+          user = User.find_by_id(params[:user_id])
+          not_found!('User') unless user
+
+          member = source.add_user(user, params[:access_level], current_user: current_user, expires_at: params[:expires_at])
 
           if !member
             not_allowed! # This currently can only be reached in EE
diff --git a/spec/requests/api/members_spec.rb b/spec/requests/api/members_spec.rb
index c621760b6c4..93e1c3a2294 100644
--- a/spec/requests/api/members_spec.rb
+++ b/spec/requests/api/members_spec.rb
@@ -231,6 +231,14 @@ describe API::Members do
         expect(response).to have_gitlab_http_status(409)
       end
 
+      it 'returns 404 when the user_id is not valid' do
+        post api("/#{source_type.pluralize}/#{source.id}/members", maintainer),
+             user_id: 0, access_level: Member::MAINTAINER
+
+        expect(response).to have_gitlab_http_status(404)
+        expect(json_response['message']).to eq('404 User Not Found')
+      end
+
       it 'returns 400 when user_id is not given' do
         post api("/#{source_type.pluralize}/#{source.id}/members", maintainer),
              access_level: Member::MAINTAINER