Index: src/docs/user/userguide/remarkup.diviner
===================================================================
--- src/docs/user/userguide/remarkup.diviner
+++ src/docs/user/userguide/remarkup.diviner
@@ -18,7 +18,7 @@
 
 These are inline styles, and can be applied to most text:
 
-  **bold** //italic// ##monospaced## `monospaced` ~~deleted~~
+  **bold** //italic// ##monospaced## `monospaced` ~~deleted~~ __underlined__
   D123 T123 rX123           # Link to Objects
   {D123} {T123}             # Link to Objects (Full Name)
   {F123}                    # Embed Images
@@ -58,9 +58,10 @@
   ##monospaced text##
   `monospaced text`
   ~~deleted text~~
+  __underlined text__
 
 Those produce **bold text**, //italic text//, ##monospaced text##,
-`monospaced text` and ~~deleted text~~, respectively.
+`monospaced text`, ~~deleted text~~, and __underlined text__, respectively.
 
 = Layout =
 
Index: src/infrastructure/markup/PhabricatorMarkupEngine.php
===================================================================
--- src/infrastructure/markup/PhabricatorMarkupEngine.php
+++ src/infrastructure/markup/PhabricatorMarkupEngine.php
@@ -455,6 +455,7 @@
     $rules[] = new PhutilRemarkupRuleBold();
     $rules[] = new PhutilRemarkupRuleItalic();
     $rules[] = new PhutilRemarkupRuleDel();
+    $rules[] = new PhutilRemarkupRuleUnderline();
 
     foreach (self::loadCustomInlineRules() as $rule) {
       $rules[] = $rule;