diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 369291a3c..f225e8394 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -1307,6 +1307,13 @@ module ApplicationHelper
             name = remove_double_quotes(identifier)
             u = User.visible.find_by("LOWER(login) = :s AND type = 'User'", :s => name.downcase)
             link = link_to_user(u, :only_path => only_path, :class => 'user-mention', :mention => true) if u
+            if u && obj && !obj.visible?(u)
+              link += content_tag(
+                'span', l(:notice_invalid_mention),
+                class: 'icon-only icon-warning',
+                title: l(:notice_invalid_mention)
+              )
+            end
           end
         end
         (leading + (link || "#{project_prefix}#{prefix}#{repo_prefix}#{sep}#{identifier}#{comment_suffix}"))
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 1584f5ef2..a927f4d87 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -195,6 +195,7 @@ en:
   notice_issue_not_closable_by_blocking_issue: "This issue cannot be closed because it is blocked by at least one open issue."
   notice_issue_not_reopenable_by_closed_parent_issue: "This issue cannot be reopened because its parent issue is closed."
   notice_invalid_watcher: "Invalid watcher: User will not receive any notifications because they do not have access to view this object."
+  notice_invalid_mention: "Invalid mention: User will not receive mention notifications because it does not have access to view this object."
 
   error_can_t_load_default_data: "Default configuration could not be loaded: %{value}"
   error_scm_not_found: "The entry or revision was not found in the repository."
diff --git a/test/helpers/application_helper_test.rb b/test/helpers/application_helper_test.rb
index 82991888d..1c14d62d4 100644
--- a/test/helpers/application_helper_test.rb
+++ b/test/helpers/application_helper_test.rb
@@ -612,6 +612,23 @@ class ApplicationHelperTest < Redmine::HelperTest
     end
   end
 
+  def test_mention_link_to_users_who_are_not_authorised_to_view_the_object
+    set_language_if_valid 'en'
+    with_settings :text_formatting => 'textile' do
+      issue = Issue.find(4)
+      assert_not issue.visible?(User.find(3))
+
+      expected_output =
+        link_to_user(User.find(3), only_path: true, class: 'user-mention', mention: true) +
+        content_tag('span', l(:notice_invalid_mention),
+                    class: 'icon-only icon-warning', title: l(:notice_invalid_mention))
+      assert_equal "<p>#{expected_output}</p>", textilizable("@#{User.find(3).login}", :object => issue)
+
+      journal = Journal.find(2)
+      assert_equal '<p><a href="#note-2">#note-2</a></p>', textilizable('#note-2', :object => journal)
+    end
+  end
+
   def test_link_to_note_within_the_same_page
     with_settings :text_formatting => 'textile' do
       issue = Issue.find(1)