diff --git a/src/applications/auth/controller/PhabricatorAuthRegisterController.php b/src/applications/auth/controller/PhabricatorAuthRegisterController.php
--- a/src/applications/auth/controller/PhabricatorAuthRegisterController.php
+++ b/src/applications/auth/controller/PhabricatorAuthRegisterController.php
@@ -342,6 +342,10 @@
         if (!strlen($value_realname) && $require_real_name) {
           $e_realname = pht('Required');
           $errors[] = pht('Real name is required.');
+        } else if ($value_realname &&
+                 !PhabricatorUser::validateRealName($value_realname)) {
+          $e_realname = pht('Invalid');
+          $errors[] = PhabricatorUser::describeValidRealName();
         } else {
           $e_realname = null;
         }
diff --git a/src/applications/people/storage/PhabricatorUser.php b/src/applications/people/storage/PhabricatorUser.php
--- a/src/applications/people/storage/PhabricatorUser.php
+++ b/src/applications/people/storage/PhabricatorUser.php
@@ -26,6 +26,7 @@
   const SESSION_TABLE = 'phabricator_session';
   const NAMETOKEN_TABLE = 'user_nametoken';
   const MAXIMUM_USERNAME_LENGTH = 64;
+  const MAXIMUM_REALNAME_LENGTH = 256;
 
   protected $userName;
   protected $realName;
@@ -550,6 +551,16 @@
     return (bool)preg_match('/^[a-zA-Z0-9._-]*[a-zA-Z0-9_-]\z/', $username);
   }
 
+  public static function describeValidRealName() {
+    return pht(
+      'Real Name must have no more than %d characters.',
+      new PhutilNumber(self::MAXIMUM_REALNAME_LENGTH));
+  }
+
+  public static function validateRealName($realname) {
+    return strlen($realname) <= self::MAXIMUM_REALNAME_LENGTH;
+  }
+
   public static function getDefaultProfileImageURI() {
     return celerity_get_resource_uri('/rsrc/image/avatar.png');
   }