diff --git a/src/activitypub/helpers.js b/src/activitypub/helpers.js
index 959c8a5419..5e86939878 100644
--- a/src/activitypub/helpers.js
+++ b/src/activitypub/helpers.js
@@ -4,6 +4,7 @@ const request = require('request-promise-native');
 const { generateKeyPairSync } = require('crypto');
 const winston = require('winston');
 const nconf = require('nconf');
+const validator = require('validator');
 
 const db = require('../database');
 const ttl = require('../cache/ttl');
@@ -68,10 +69,26 @@ Helpers.generateKeys = async (uid) => {
 	return { publicKey, privateKey };
 };
 
-Helpers.resolveLocalUid = async (id) => {
-	const [slug, host] = id.split('@');
+Helpers.resolveLocalUid = async (input) => {
+	let slug;
 
-	if (id.indexOf('@') === -1 || host !== nconf.get('url_parsed').host) {
+	if (validator.isURL(input, {
+		require_protocol: true,
+		require_host: true,
+		require_tld: false,
+		protocols: ['https'],
+		require_valid_protocol: true,
+	})) {
+		const { host, pathname } = new URL(input);
+
+		if (host === nconf.get('url_parsed').host) {
+			slug = pathname.split('/').filter(Boolean)[1];
+		} else {
+			throw new Error('[[activitypub:invalid-id]]');
+		}
+	} else if (input.indexOf('@') !== -1) { // Webfinger
+		([slug] = input.replace(/^acct:/, '').split('@'));
+	} else {
 		throw new Error('[[activitypub:invalid-id]]');
 	}
 
diff --git a/test/activitypub.js b/test/activitypub.js
index d59fe1cbc1..7b6748bea2 100644
--- a/test/activitypub.js
+++ b/test/activitypub.js
@@ -100,6 +100,53 @@ describe('ActivityPub integration', () => {
 		});
 	});
 
+	describe.only('Helpers', () => {
+		describe('.query()', () => {
+
+		});
+
+		describe('.generateKeys()', () => {
+
+		});
+
+		describe('.resolveLocalUid()', () => {
+			let uid;
+			let slug;
+
+			beforeEach(async () => {
+				slug = slugify(utils.generateUUID().slice(0, 8));
+				uid = await user.create({ username: slug });
+			});
+
+			it('should throw when an invalid input is passed in', async () => {
+				await assert.rejects(
+					activitypub.helpers.resolveLocalUid('ncl28h3qwhoiclwnevoinw3u'),
+					{ message: '[[activitypub:invalid-id]]' }
+				);
+			});
+
+			it('should return null when valid input is passed but does not resolve', async () => {
+				const uid = await activitypub.helpers.resolveLocalUid(`acct:foobar@${nconf.get('url_parsed').host}`);
+				assert.strictEqual(uid, null);
+			});
+
+			it('should resolve to a local uid when given a webfinger-style string', async () => {
+				const found = await activitypub.helpers.resolveLocalUid(`acct:${slug}@${nconf.get('url_parsed').host}`);
+				assert.strictEqual(found, uid);
+			});
+
+			it('should resolve even without the "acct:" prefix', async () => {
+				const found = await activitypub.helpers.resolveLocalUid(`${slug}@${nconf.get('url_parsed').host}`);
+				assert.strictEqual(found, uid);
+			});
+
+			it('should resolve when passed a full URL', async () => {
+				const found = await activitypub.helpers.resolveLocalUid(`${nconf.get('url')}/user/${slug}`);
+				assert.strictEqual(found, uid);
+			});
+		});
+	});
+
 	describe('ActivityPub screener middleware', () => {
 		let uid;
 		let slug;