diff --git a/src/controllers/write/topics.js b/src/controllers/write/topics.js
index 5bca992074..f26964c13d 100644
--- a/src/controllers/write/topics.js
+++ b/src/controllers/write/topics.js
@@ -3,6 +3,7 @@
 const db = require('../../database');
 const api = require('../../api');
 const topics = require('../../topics');
+const activitypub = require('../../activitypub');
 
 const helpers = require('../helpers');
 const middleware = require('../../middleware');
@@ -222,6 +223,7 @@ Topics.getCrossposts = async (req, res) => {
 Topics.crosspost = async (req, res) => {
 	const { cid } = req.body;
 	const crossposts = await topics.crossposts.add(req.params.tid, cid, req.uid);
+	await activitypub.out.announce.topic(req.params.tid, req.uid);
 
 	helpers.formatApiResponse(200, res, { crossposts });
 };
@@ -229,6 +231,7 @@ Topics.crosspost = async (req, res) => {
 Topics.uncrosspost = async (req, res) => {
 	const { cid } = req.body;
 	const crossposts = await topics.crossposts.remove(req.params.tid, cid, req.uid);
+	await activitypub.out.undo.announce('uid', req.uid, req.parms.tid);
 
 	helpers.formatApiResponse(200, res, { crossposts });
 };