diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -4506,6 +4506,7 @@
     'PhabricatorProjectTriggerInvalidRule' => 'applications/project/trigger/PhabricatorProjectTriggerInvalidRule.php',
     'PhabricatorProjectTriggerListController' => 'applications/project/controller/trigger/PhabricatorProjectTriggerListController.php',
     'PhabricatorProjectTriggerManiphestOwnerRule' => 'applications/project/trigger/PhabricatorProjectTriggerManiphestOwnerRule.php',
+    'PhabricatorProjectTriggerManiphestOwnerUnassignedRule' => 'applications/project/trigger/PhabricatorProjectTriggerManiphestOwnerUnassignedRule.php',
     'PhabricatorProjectTriggerManiphestPriorityRule' => 'applications/project/trigger/PhabricatorProjectTriggerManiphestPriorityRule.php',
     'PhabricatorProjectTriggerManiphestStatusRule' => 'applications/project/trigger/PhabricatorProjectTriggerManiphestStatusRule.php',
     'PhabricatorProjectTriggerNameTransaction' => 'applications/project/xaction/trigger/PhabricatorProjectTriggerNameTransaction.php',
@@ -11165,6 +11166,7 @@
     'PhabricatorProjectTriggerInvalidRule' => 'PhabricatorProjectTriggerRule',
     'PhabricatorProjectTriggerListController' => 'PhabricatorProjectTriggerController',
     'PhabricatorProjectTriggerManiphestOwnerRule' => 'PhabricatorProjectTriggerRule',
+    'PhabricatorProjectTriggerManiphestOwnerUnassignedRule' => 'PhabricatorProjectTriggerManiphestOwnerRule',
     'PhabricatorProjectTriggerManiphestPriorityRule' => 'PhabricatorProjectTriggerRule',
     'PhabricatorProjectTriggerManiphestStatusRule' => 'PhabricatorProjectTriggerRule',
     'PhabricatorProjectTriggerNameTransaction' => 'PhabricatorProjectTriggerTransactionType',
diff --git a/src/applications/project/trigger/PhabricatorProjectTriggerManiphestOwnerRule.php b/src/applications/project/trigger/PhabricatorProjectTriggerManiphestOwnerRule.php
--- a/src/applications/project/trigger/PhabricatorProjectTriggerManiphestOwnerRule.php
+++ b/src/applications/project/trigger/PhabricatorProjectTriggerManiphestOwnerRule.php
@@ -1,6 +1,6 @@
 <?php
 
-final class PhabricatorProjectTriggerManiphestOwnerRule
+class PhabricatorProjectTriggerManiphestOwnerRule
   extends PhabricatorProjectTriggerRule {
 
   const TRIGGERTYPE = 'task.owner';
@@ -13,7 +13,7 @@
     return $this->getDatasource()->getWireTokens($this->getValue());
   }
 
-  private function convertTokenizerValueToOwner($value) {
+  protected function convertTokenizerValueToOwner($value) {
     $value = head($value);
     if ($value === PhabricatorPeopleNoOwnerDatasource::FUNCTION_TOKEN) {
       $value = null;
diff --git a/src/applications/project/trigger/PhabricatorProjectTriggerManiphestOwnerUnassignedRule.php b/src/applications/project/trigger/PhabricatorProjectTriggerManiphestOwnerUnassignedRule.php
new file mode 100644
--- /dev/null
+++ b/src/applications/project/trigger/PhabricatorProjectTriggerManiphestOwnerUnassignedRule.php
@@ -0,0 +1,42 @@
+<?php
+
+final class PhabricatorProjectTriggerManiphestOwnerUnassignedRule
+  extends PhabricatorProjectTriggerManiphestOwnerRule {
+
+  const TRIGGERTYPE = 'task.owner.unassigned';
+
+  public function getSelectControlName() {
+    return pht('Assign unowned task to');
+  }
+
+  protected function newDropTransactions($object, $value) {
+    if ($object->getOwnerPHID()) { // do nothing if task already has assignee
+      return array();
+    } else {
+      $value = $this->convertTokenizerValueToOwner($value);
+      return array(
+        $this->newTransaction()
+          ->setTransactionType(ManiphestTaskOwnerTransaction::TRANSACTIONTYPE)
+          ->setNewValue($value),
+      );
+    }
+  }
+
+   public function getRuleViewDescription($value) {
+    if (head($value) === PhabricatorViewerDatasource::FUNCTION_TOKEN) {
+      return pht('Assign unowned task to user moving the task.');
+    } else if ($value) {
+      $value = $this->convertTokenizerValueToOwner($value);
+       return pht(
+         'Assign unowned task to %s.',
+          phutil_tag(
+            'strong',
+            array(),
+            $this->getViewer()
+              ->renderHandle($value)
+              ->render()));
+    } else { // !$value
+      return pht('Unassign task.');
+    }
+  }
+}