diff --git a/src/applications/maniphest/xaction/ManiphestTaskTitleTransaction.php b/src/applications/maniphest/xaction/ManiphestTaskTitleTransaction.php
--- a/src/applications/maniphest/xaction/ManiphestTaskTitleTransaction.php
+++ b/src/applications/maniphest/xaction/ManiphestTaskTitleTransaction.php
@@ -78,6 +78,12 @@
           $xaction);
         continue;
       }
+      if (mb_strlen($new) > 255) {
+        $errors[] = $this->newInvalidError(
+          pht('Task title must be less than 256 characters.'),
+          $xaction);
+        continue;
+      }
     }
 
     if (!$errors) {