diff --git a/lib/shared/messages/reaction-message-spec.js b/lib/shared/messages/reaction-message-spec.js
--- a/lib/shared/messages/reaction-message-spec.js
+++ b/lib/shared/messages/reaction-message-spec.js
@@ -25,7 +25,7 @@
   MessageTitleParam,
   NotificationTextsParams,
 } from './message-spec';
-import { assertSingleMessageInfo } from './utils';
+import { assertSingleMessageInfo, joinResult } from './utils';
 
 export const reactionMessageSpec: MessageSpec<
   ReactionMessageData,
@@ -193,5 +193,14 @@
     };
   },
 
+  notificationCollapseKey(rawMessageInfo: RawReactionMessageInfo): string {
+    return joinResult(
+      rawMessageInfo.type,
+      rawMessageInfo.threadID,
+      rawMessageInfo.creatorID,
+      rawMessageInfo.targetMessageID,
+    );
+  },
+
   generatesNotifs: true,
 });