diff --git a/app/assets/javascripts/jstoolbar/common_mark.js b/app/assets/javascripts/jstoolbar/common_mark.js
index f9e5ba7cd..bda41b532 100644
--- a/app/assets/javascripts/jstoolbar/common_mark.js
+++ b/app/assets/javascripts/jstoolbar/common_mark.js
@@ -26,6 +26,16 @@ jsToolBar.prototype.elements.em = {
   }
 }
 
+// ins
+jsToolBar.prototype.elements.ins = {
+  type: 'button',
+  title: 'Underline',
+  shortcut: 'u',
+  fn: {
+    wiki: function() { this.singleTag('<u>', '</u>') }
+  }
+}
+
 // del
 jsToolBar.prototype.elements.del = {
   type: 'button',
diff --git a/lib/redmine/wiki_formatting/common_mark/sanitization_filter.rb b/lib/redmine/wiki_formatting/common_mark/sanitization_filter.rb
index c689f6d9b..23fe73a58 100644
--- a/lib/redmine/wiki_formatting/common_mark/sanitization_filter.rb
+++ b/lib/redmine/wiki_formatting/common_mark/sanitization_filter.rb
@@ -111,6 +111,9 @@ module Redmine
             end
           }
 
+          # Allow `u`` element to enable underline
+          allowlist[:elements].push('u')
+
           allowlist
         end
       end