diff --git a/src/applications/diffusion/remarkup/__tests__/DiffusionCommitRemarkupRuleTestCase.php b/src/applications/diffusion/remarkup/__tests__/DiffusionCommitRemarkupRuleTestCase.php
--- a/src/applications/diffusion/remarkup/__tests__/DiffusionCommitRemarkupRuleTestCase.php
+++ b/src/applications/diffusion/remarkup/__tests__/DiffusionCommitRemarkupRuleTestCase.php
@@ -121,6 +121,25 @@
           ),
         ),
       ),
+
+      // After an "@", we should not be recognizing references because these
+      // are username mentions.
+      'deadbeef' => array(
+        'embed' => array(
+        ),
+        'ref' => array(
+          array(
+            'offset' => 0,
+            'id' => 'deadbeef',
+          ),
+        ),
+      ),
+      '@deadbeef' => array(
+        'embed' => array(
+        ),
+        'ref' => array(
+        ),
+      ),
     );
 
     foreach ($cases as $input => $expect) {
diff --git a/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php b/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php
--- a/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php
+++ b/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php
@@ -209,13 +209,14 @@
       $boundary = '\\B';
     }
 
-    // The "(?<![#-])" prevents us from linking "#abcdef" or similar, and
-    // "ABC-T1" (see T5714).
+    // The "(?<![#@-])" prevents us from linking "#abcdef" or similar, and
+    // "ABC-T1" (see T5714), and from matching "@T1" as a task (it is a user)
+    // (see T9479).
 
     // The "\b" allows us to link "(abcdef)" or similar without linking things
     // in the middle of words.
 
-    return '((?<![#-])'.$boundary.$prefix.'('.$id.')(?:#([-\w\d]+))?(?!\w))u';
+    return '((?<![#@-])'.$boundary.$prefix.'('.$id.')(?:#([-\w\d]+))?(?!\w))u';
   }