diff --git a/src/api/activitypub.js b/src/api/activitypub.js
index dd0cd4a19f..463c57a4bb 100644
--- a/src/api/activitypub.js
+++ b/src/api/activitypub.js
@@ -223,10 +223,37 @@ activitypubApi.flag = enabledCheck(async (caller, flag) => {
 	if (flag.type === 'post' && activitypub.helpers.isUri(flag.targetUid)) {
 		reportedIds.push(flag.targetUid);
 	}
-	const reason = flag.reports.filter(report => report.reporter.uid === caller.uid).at(-1);
+	const reason = flag.reason ||
+		(flag.reports && flag.reports.filter(report => report.reporter.uid === caller.uid).at(-1).value);
 	await activitypub.send('uid', caller.uid, reportedIds, {
+		id: `${nconf.get('url')}/${flag.type}/${encodeURIComponent(flag.targetId)}#activity/flag/${caller.uid}`,
 		type: 'Flag',
 		object: reportedIds,
-		content: reason ? reason.value : undefined,
+		content: reason,
 	});
+	await db.sortedSetAdd(`flag:${flag.flagId}:remote`, Date.now(), caller.uid);
+});
+
+activitypubApi.undo.flag = enabledCheck(async (caller, flag) => {
+	if (!activitypub.helpers.isUri(flag.targetId)) {
+		return;
+	}
+	const reportedIds = [flag.targetId];
+	if (flag.type === 'post' && activitypub.helpers.isUri(flag.targetUid)) {
+		reportedIds.push(flag.targetUid);
+	}
+	const reason = flag.reason ||
+		(flag.reports && flag.reports.filter(report => report.reporter.uid === caller.uid).at(-1).value);
+	await activitypub.send('uid', caller.uid, reportedIds, {
+		id: `${nconf.get('url')}/${flag.type}/${encodeURIComponent(flag.targetId)}#activity/undo:flag/${caller.uid}`,
+		type: 'Undo',
+		object: {
+			id: `${nconf.get('url')}/${flag.type}/${encodeURIComponent(flag.targetId)}#activity/flag/${caller.uid}`,
+			actor: `${nconf.get('url')}/uid/${caller.uid}`,
+			type: 'Flag',
+			object: reportedIds,
+			content: reason,
+		},
+	});
+	await db.sortedSetRemove(`flag:${flag.flagId}:remote`, caller.uid);
 });
diff --git a/src/flags.js b/src/flags.js
index d62f9e00ba..864057729c 100644
--- a/src/flags.js
+++ b/src/flags.js
@@ -418,7 +418,7 @@ Flags.create = async function (type, id, uid, reason, timestamp, forceFlag = fal
 	if (targetFlagged) {
 		const flagId = await Flags.getFlagIdByTarget(type, id);
 		await Promise.all([
-			Flags.addReport(flagId, type, id, uid, reason, timestamp),
+			Flags.addReport(flagId, type, id, uid, reason, timestamp, targetUid, notifyRemote),
 			Flags.update(flagId, uid, {
 				state: 'open',
 				report: 'added',
@@ -439,7 +439,7 @@ Flags.create = async function (type, id, uid, reason, timestamp, forceFlag = fal
 			targetUid: targetUid,
 			datetime: timestamp,
 		}),
-		Flags.addReport(flagId, type, id, uid, reason, timestamp),
+		Flags.addReport(flagId, type, id, uid, reason, timestamp, targetUid, notifyRemote),
 		db.sortedSetAdd('flags:datetime', timestamp, flagId), // by time, the default
 		db.sortedSetAdd(`flags:byType:${type}`, timestamp, flagId), // by flag type
 		db.sortedSetIncrBy('flags:byTarget', 1, [type, id].join(':')), // by flag target (score is count)
@@ -478,7 +478,7 @@ Flags.create = async function (type, id, uid, reason, timestamp, forceFlag = fal
 
 	if (notifyRemote && activitypub.helpers.isUri(id)) {
 		const caller = await user.getUserData(uid);
-		activitypubApi.flag(caller, flagObj);
+		activitypubApi.flag(caller, { ...flagObj, reason });
 	}
 
 	plugins.hooks.fire('action:flags.create', { flag: flagObj });
@@ -528,6 +528,13 @@ Flags.purge = async function (flagIds) {
 			'flags:byTarget',
 			flagData.map(flagObj => [flagObj.type, flagObj.targetId].join(':'))
 		),
+		flagData.flatMap(
+			async (flagObj, i) => allReporterUids[i].map(async (uid) => {
+				if (await db.isSortedSetMember(`flag:${flagObj.flagId}:remote`, uid)) {
+					await activitypubApi.undo.flag({ uid }, flagObj);
+				}
+			})
+		),
 	]);
 };
 
@@ -553,7 +560,7 @@ Flags.getReports = async function (flagId) {
 };
 
 // Not meant to be called directly, call Flags.create() instead.
-Flags.addReport = async function (flagId, type, id, uid, reason, timestamp) {
+Flags.addReport = async function (flagId, type, id, uid, reason, timestamp, targetUid, notifyRemote) {
 	await db.sortedSetAddBulk([
 		[`flags:byReporter:${uid}`, timestamp, flagId],
 		[`flag:${flagId}:reports`, timestamp, [uid, reason].join(';')],
@@ -561,6 +568,10 @@ Flags.addReport = async function (flagId, type, id, uid, reason, timestamp) {
 		['flags:hash', flagId, [type, id, uid].join(':')],
 	]);
 
+	if (notifyRemote && activitypub.helpers.isUri(id)) {
+		await activitypubApi.flag({ uid }, { flagId, type, targetId: id, targetUid, uid, reason, timestamp });
+	}
+
 	plugins.hooks.fire('action:flags.addReport', { flagId, type, id, uid, reason, timestamp });
 };
 
@@ -586,6 +597,11 @@ Flags.rescindReport = async (type, id, uid) => {
 		throw new Error('[[error:cant-locate-flag-report]]');
 	}
 
+	if (await db.isSortedSetMember(`flag:${flagId}:remote`, uid)) {
+		const flag = await Flags.get(flagId);
+		await activitypubApi.undo.flag({ uid }, flag);
+	}
+
 	await db.sortedSetRemoveBulk([
 		[`flags:byReporter:${uid}`, flagId],
 		[`flag:${flagId}:reports`, [uid, reason].join(';')],