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
@@ -1346,6 +1346,7 @@
     'HarbormasterWaitForPreviousBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterWaitForPreviousBuildStepImplementation.php',
     'HarbormasterWorker' => 'applications/harbormaster/worker/HarbormasterWorker.php',
     'HarbormasterWorkingCopyArtifact' => 'applications/harbormaster/artifact/HarbormasterWorkingCopyArtifact.php',
+    'HeraldActingUserField' => 'applications/herald/field/HeraldActingUserField.php',
     'HeraldAction' => 'applications/herald/action/HeraldAction.php',
     'HeraldActionGroup' => 'applications/herald/action/HeraldActionGroup.php',
     'HeraldActionRecord' => 'applications/herald/storage/HeraldActionRecord.php',
@@ -6589,6 +6590,7 @@
     'HarbormasterWaitForPreviousBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterWorker' => 'PhabricatorWorker',
     'HarbormasterWorkingCopyArtifact' => 'HarbormasterDrydockLeaseArtifact',
+    'HeraldActingUserField' => 'HeraldField',
     'HeraldAction' => 'Phobject',
     'HeraldActionGroup' => 'HeraldGroup',
     'HeraldActionRecord' => 'HeraldDAO',
diff --git a/src/applications/diffusion/engine/DiffusionCommitHookEngine.php b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
--- a/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
+++ b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
@@ -297,7 +297,11 @@
       return;
     }
 
-    $adapter_template->setHookEngine($this);
+    $viewer = $this->getViewer();
+
+    $adapter_template
+      ->setHookEngine($this)
+      ->setActingAsPHID($viewer->getPHID());
 
     $engine = new HeraldEngine();
     $rules = null;
diff --git a/src/applications/herald/adapter/HeraldAdapter.php b/src/applications/herald/adapter/HeraldAdapter.php
--- a/src/applications/herald/adapter/HeraldAdapter.php
+++ b/src/applications/herald/adapter/HeraldAdapter.php
@@ -40,6 +40,7 @@
   private $forbiddenActions = array();
   private $viewer;
   private $mustEncryptReasons = array();
+  private $actingAsPHID;
 
   public function getEmailPHIDs() {
     return array_values($this->emailPHIDs);
@@ -49,6 +50,15 @@
     return array_values($this->forcedEmailPHIDs);
   }
 
+  final public function setActingAsPHID($acting_as_phid) {
+    $this->actingAsPHID = $acting_as_phid;
+    return $this;
+  }
+
+  final public function getActingAsPHID() {
+    return $this->actingAsPHID;
+  }
+
   public function addEmailPHID($phid, $force) {
     $this->emailPHIDs[$phid] = $phid;
     if ($force) {
diff --git a/src/applications/herald/controller/HeraldTestConsoleController.php b/src/applications/herald/controller/HeraldTestConsoleController.php
--- a/src/applications/herald/controller/HeraldTestConsoleController.php
+++ b/src/applications/herald/controller/HeraldTestConsoleController.php
@@ -41,6 +41,7 @@
 
     $adapter
       ->setIsNewObject(false)
+      ->setActingAsPHID($viewer->getPHID())
       ->setViewer($viewer);
 
     $rules = id(new HeraldRuleQuery())
diff --git a/src/applications/herald/field/HeraldActingUserField.php b/src/applications/herald/field/HeraldActingUserField.php
new file mode 100644
--- /dev/null
+++ b/src/applications/herald/field/HeraldActingUserField.php
@@ -0,0 +1,32 @@
+<?php
+
+final class HeraldActingUserField
+  extends HeraldField {
+
+  const FIELDCONST = 'herald.acting-user';
+
+  public function getHeraldFieldName() {
+    return pht('Acting user');
+  }
+
+  public function getHeraldFieldValue($object) {
+    return $this->getAdapter()->getActingAsPHID();
+  }
+
+  protected function getHeraldFieldStandardType() {
+    return self::STANDARD_PHID;
+  }
+
+  protected function getDatasource() {
+    return new PhabricatorPeopleDatasource();
+  }
+
+  public function supportsObject($object) {
+    return true;
+  }
+
+  public function getFieldGroupKey() {
+    return HeraldEditFieldGroup::FIELDGROUPKEY;
+  }
+
+}
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -3254,6 +3254,7 @@
     $adapter = $this->buildHeraldAdapter($object, $xactions)
       ->setContentSource($this->getContentSource())
       ->setIsNewObject($this->getIsNewObject())
+      ->setActingAsPHID($this->getActingAsPHID())
       ->setAppliedTransactions($xactions);
 
     if ($this->getApplicationEmail()) {