diff --git a/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php b/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
--- a/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
+++ b/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
@@ -234,6 +234,15 @@
         '[Differential]')
         ->setDescription(pht('Subject prefix for Differential mail.')),
       $this->newOption(
+        'metamta.differential.subject-template',
+        'string',
+        'D{$id}: {$title}')
+        ->setSummary(pht('Subject template for Differential mail.'))
+        ->setDescription(pht('Available keys: id, title, repository, branch'))
+        ->addExample(
+          '[${repository}/${branch}] D${id}: ${title}',
+          pht('Use repository and branch')),
+      $this->newOption(
         'metamta.differential.attach-patches',
         'bool',
         false)
diff --git a/src/applications/differential/editor/DifferentialTransactionEditor.php b/src/applications/differential/editor/DifferentialTransactionEditor.php
--- a/src/applications/differential/editor/DifferentialTransactionEditor.php
+++ b/src/applications/differential/editor/DifferentialTransactionEditor.php
@@ -1081,6 +1081,11 @@
     return PhabricatorEnv::getEnvConfig('metamta.differential.subject-prefix');
   }
 
+  private function getMailSubjectTemplate() {
+    return PhabricatorEnv::getEnvConfig(
+      'metamta.differential.subject-template');
+  }
+
   protected function getMailThreadID(PhabricatorLiskDAO $object) {
     // This is nonstandard, but retains threading with older messages.
     $phid = $object->getPHID();
@@ -1097,15 +1102,66 @@
     $title = $object->getTitle();
 
     $original_title = $object->getOriginalTitle();
+    $branch = $object->getActiveDiff()->getBranch();
+    $repository_name = $object->getRepository()->getName();
+
+    $base_variables = array(
+      'id' => $id,
+      'branch' => $branch,
+      'repository' => $repository_name
+    );
 
-    $subject = "D{$id}: {$title}";
-    $thread_topic = "D{$id}: {$original_title}";
+    $subject = $this->mergeVariables($this->getMailSubjectTemplate(),
+      array_merge($base_variables, array(
+        'title' => $title
+    )));
+
+    $thread_topic = $this->mergeVariables($this->getMailSubjectTemplate(),
+      array_merge($base_variables, array(
+        'title' => $original_title
+      )));
 
     return id(new PhabricatorMetaMTAMail())
       ->setSubject($subject)
       ->addHeader('Thread-Topic', $thread_topic);
   }
 
+  /**
+   * Convert a user-provided string with variables in it, like:
+   *
+   *   ${project}/${branch}
+   *
+   * ...into a string with variables merged into it safely:
+   *
+   *   myproject/mybranch
+   *
+   * Maybe move to libphutil?
+   * @param string User-provided pattern string containing `${variables}`.
+   * @param dict   List of available replacement variables.
+   * @return string String with variables replaced safely into it.
+   *
+   */
+  protected function mergeVariables($pattern, array $variables) {
+    $regexp = '/\\$\\{(?P<name>[a-z\\.]+)\\}/';
+
+    $matches = null;
+    preg_match_all($regexp, $pattern, $matches);
+
+    $argv = array();
+    foreach ($matches['name'] as $name) {
+      if (!array_key_exists($name, $variables)) {
+        throw new Exception(pht("No such variable '%s'!", $name));
+      }
+      $argv[] = $variables[$name];
+    }
+
+    $pattern = str_replace('%', '%%', $pattern);
+    $pattern = preg_replace($regexp, '%s', $pattern);
+
+    return vsprintf($pattern, $argv);
+  }
+
+
   protected function buildMailBody(
     PhabricatorLiskDAO $object,
     array $xactions) {