diff --git a/public/src/modules/flags.js b/public/src/modules/flags.js
index 845a605f01..a105e22181 100644
--- a/public/src/modules/flags.js
+++ b/public/src/modules/flags.js
@@ -8,6 +8,8 @@ define('flags', ['hooks', 'components', 'api', 'alerts'], function (hooks, compo
 	let flagReason;
 
 	Flag.showFlagModal = function (data) {
+		data.remote = URL.canParse(data.id) ? new URL(data.id).hostname : false;
+
 		app.parseAndTranslate('modals/flag', data, function (html) {
 			flagModal = html;
 			flagModal.on('hidden.bs.modal', function () {
@@ -35,18 +37,21 @@ define('flags', ['hooks', 'components', 'api', 'alerts'], function (hooks, compo
 				if (selected.attr('id') === 'flag-reason-other') {
 					reason = flagReason.val();
 				}
-				createFlag(data.type, data.id, reason);
+				const notifyRemote = $('input[name="flag-notify-remote"]').is(':checked');
+				createFlag(data.type, data.id, reason, notifyRemote);
 			});
 
 			flagModal.on('click', '#flag-reason-other', function () {
 				flagReason.focus();
 			});
 
+
 			flagModal.modal('show');
 			hooks.fire('action:flag.showModal', {
 				modalEl: flagModal,
 				type: data.type,
 				id: data.id,
+				remote: data.remote,
 			});
 
 			flagModal.find('#flag-reason-custom').on('keyup blur change', checkFlagButtonEnable);
@@ -63,7 +68,7 @@ define('flags', ['hooks', 'components', 'api', 'alerts'], function (hooks, compo
 	};
 
 	Flag.rescindByType = function (type, id) {
-		api.del(`/flags/${type}/${id}/report`).then(() => {
+		api.del(`/flags/${type}/${encodeURIComponent(id)}/report`).then(() => {
 			alerts.success('[[flags:rescinded]]');
 			hooks.fire('action:flag.rescinded', { type: type, id: id });
 			if (type === 'post') {
@@ -88,11 +93,11 @@ define('flags', ['hooks', 'components', 'api', 'alerts'], function (hooks, compo
 		}).catch(alerts.error);
 	};
 
-	function createFlag(type, id, reason) {
+	function createFlag(type, id, reason, notifyRemote = false) {
 		if (!type || !id || !reason) {
 			return;
 		}
-		const data = { type: type, id: id, reason: reason };
+		const data = { type: type, id: id, reason: reason, notifyRemote: notifyRemote };
 		api.post('/flags', data, function (err, flagId) {
 			if (err) {
 				return alerts.error(err);
diff --git a/src/api/activitypub.js b/src/api/activitypub.js
index 6e92763c45..dd0cd4a19f 100644
--- a/src/api/activitypub.js
+++ b/src/api/activitypub.js
@@ -214,3 +214,19 @@ activitypubApi.undo.like = enabledCheck(async (caller, { pid }) => {
 		},
 	});
 });
+
+activitypubApi.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.reports.filter(report => report.reporter.uid === caller.uid).at(-1);
+	await activitypub.send('uid', caller.uid, reportedIds, {
+		type: 'Flag',
+		object: reportedIds,
+		content: reason ? reason.value : undefined,
+	});
+});
diff --git a/src/api/flags.js b/src/api/flags.js
index 78fe35bb51..ff30978982 100644
--- a/src/api/flags.js
+++ b/src/api/flags.js
@@ -11,7 +11,7 @@ flagsApi.create = async (caller, data) => {
 		throw new Error('[[error:invalid-data]]');
 	}
 
-	const { type, id, reason } = data;
+	const { type, id, reason, notifyRemote } = data;
 
 	await flags.validate({
 		uid: caller.uid,
@@ -19,7 +19,7 @@ flagsApi.create = async (caller, data) => {
 		id: id,
 	});
 
-	const flagObj = await flags.create(type, id, caller.uid, reason);
+	const flagObj = await flags.create(type, id, caller.uid, reason, undefined, undefined, notifyRemote);
 	flags.notify(flagObj, caller.uid);
 
 	return flagObj;
diff --git a/src/controllers/write/flags.js b/src/controllers/write/flags.js
index cb81ee10ac..2af5d7e9b4 100644
--- a/src/controllers/write/flags.js
+++ b/src/controllers/write/flags.js
@@ -7,8 +7,8 @@ const helpers = require('../helpers');
 const Flags = module.exports;
 
 Flags.create = async (req, res) => {
-	const { type, id, reason } = req.body;
-	const flagObj = await api.flags.create(req, { type, id, reason });
+	const { type, id, reason, notifyRemote } = req.body;
+	const flagObj = await api.flags.create(req, { type, id, reason, notifyRemote });
 	helpers.formatApiResponse(200, res, await user.isPrivileged(req.uid) ? flagObj : undefined);
 };
 
@@ -36,12 +36,10 @@ Flags.rescind = async (req, res) => {
 	await api.flags.rescind(req, { flagId: req.params.flagId });
 	helpers.formatApiResponse(200, res);
 };
-
 Flags.rescindPost = async (req, res) => {
 	await api.flags.rescindPost(req, { pid: req.params.pid });
 	helpers.formatApiResponse(200, res);
 };
-
 Flags.rescindUser = async (req, res) => {
 	await api.flags.rescindUser(req, { uid: req.params.uid });
 	helpers.formatApiResponse(200, res);
diff --git a/src/flags.js b/src/flags.js
index 833bb7edd8..d62f9e00ba 100644
--- a/src/flags.js
+++ b/src/flags.js
@@ -4,6 +4,8 @@ const _ = require('lodash');
 const winston = require('winston');
 const validator = require('validator');
 
+const activitypub = require('./activitypub');
+const activitypubApi = require('./api/activitypub');
 const db = require('./database');
 const user = require('./user');
 const groups = require('./groups');
@@ -389,7 +391,7 @@ Flags.deleteNote = async function (flagId, datetime) {
 	await db.sortedSetRemove(`flag:${flagId}:notes`, note[0]);
 };
 
-Flags.create = async function (type, id, uid, reason, timestamp, forceFlag = false) {
+Flags.create = async function (type, id, uid, reason, timestamp, forceFlag = false, notifyRemote = false) {
 	let doHistoryAppend = false;
 	if (!timestamp) {
 		timestamp = Date.now();
@@ -474,6 +476,11 @@ Flags.create = async function (type, id, uid, reason, timestamp, forceFlag = fal
 
 	const flagObj = await Flags.get(flagId);
 
+	if (notifyRemote && activitypub.helpers.isUri(id)) {
+		const caller = await user.getUserData(uid);
+		activitypubApi.flag(caller, flagObj);
+	}
+
 	plugins.hooks.fire('action:flags.create', { flag: flagObj });
 	return flagObj;
 };
@@ -681,6 +688,14 @@ Flags.targetExists = async function (type, id) {
 	if (type === 'post') {
 		return await posts.exists(id);
 	} else if (type === 'user') {
+		if (activitypub.helpers.isUri(id)) {
+			try {
+				const actor = await activitypub.get('uid', 0, id);
+				return !!actor;
+			} catch (_) {
+				return false;
+			}
+		}
 		return await user.exists(id);
 	}
 	throw new Error('[[error:invalid-data]]');
diff --git a/src/user/data.js b/src/user/data.js
index 86469b0f05..91b6448c60 100644
--- a/src/user/data.js
+++ b/src/user/data.js
@@ -353,7 +353,8 @@ module.exports = function (User) {
 	};
 
 	User.setUserFields = async function (uid, data) {
-		await db.setObject(`user:${uid}`, data);
+		const userKey = isFinite(uid) ? `user:${uid}` : `userRemote:${uid}`;
+		await db.setObject(userKey, data);
 		for (const [field, value] of Object.entries(data)) {
 			plugins.hooks.fire('action:user.set', { uid, field, value, type: 'set' });
 		}
diff --git a/src/views/modals/flag.tpl b/src/views/modals/flag.tpl
index 3b101138e4..c49c5e76b7 100644
--- a/src/views/modals/flag.tpl
+++ b/src/views/modals/flag.tpl
@@ -31,9 +31,17 @@
 						</label>
 					</div>
 				</div>
-				<div class="mb-3">
+				<div class="mb-2">
 					<textarea class="form-control" id="flag-reason-custom" placeholder="[[flags:modal-reason-custom]]" disabled="disabled"></textarea>
 				</div>
+				{{{ if remote }}}
+				<div class="form-check form-switch mb-3">
+					<input class="form-check-input" type="checkbox" name="flag-notify-remote" checked="checked">
+					<label class="form-check-label text-sm" for="flag-notify-remote">
+						[[flags:modal-notify-remote, {remote}]]
+					</label>
+				</div>
+				{{{ end }}}
 
 				<button type="button" class="btn btn-primary" id="flag-post-commit" disabled>[[flags:modal-submit]]</button>
 			</div>