diff --git a/app/javascript/mastodon/features/compose/util/counter.js b/app/javascript/mastodon/features/compose/util/counter.js
index ec2431096b..e59f9c1644 100644
--- a/app/javascript/mastodon/features/compose/util/counter.js
+++ b/app/javascript/mastodon/features/compose/util/counter.js
@@ -1,9 +1,20 @@
+import regexSupplant from 'twitter-text/dist/lib/regexSupplant';
+import validDomain from 'twitter-text/dist/regexp/validDomain';
+
 import { urlRegex } from './url_regex';
 
 const urlPlaceholder = '$2xxxxxxxxxxxxxxxxxxxxxxx';
 
+const validMention = regexSupplant(
+  '(^|[^/\\w])@(([a-z0-9_]+)@(#{validDomain}))',
+  {
+    validDomain,
+  },
+  'ig'
+);
+
 export function countableText(inputText) {
   return inputText
     .replace(urlRegex, urlPlaceholder)
-    .replace(/(^|[^/\w])@(([a-z0-9_]+)@[a-z0-9.-]+[a-z0-9]+)/ig, '$1@$3');
+    .replace(validMention, '$1@$3');
 }
diff --git a/app/models/account.rb b/app/models/account.rb
index 53bf2407e8..6f8c8f4c6f 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -71,7 +71,7 @@ class Account < ApplicationRecord
   INSTANCE_ACTOR_ID = -99
 
   USERNAME_RE   = /[a-z0-9_]+([.-]+[a-z0-9_]+)*/i
-  MENTION_RE    = %r{(?<![=/[:word:]])@((#{USERNAME_RE})(?:@[[:word:]]+([.-]+[[:word:]]+)*)?)}
+  MENTION_RE    = %r{(?<![=/[:word:]])@((#{USERNAME_RE})(?:@#{Twitter::TwitterText::Regex[:valid_domain]})?)}
   URL_PREFIX_RE = %r{\Ahttp(s?)://[^/]+}
   USERNAME_ONLY_RE = /\A#{USERNAME_RE}\z/i
   USERNAME_LENGTH_LIMIT = 30
diff --git a/spec/models/account_spec.rb b/spec/models/account_spec.rb
index 06b7b78e64..42f2e3679e 100644
--- a/spec/models/account_spec.rb
+++ b/spec/models/account_spec.rb
@@ -431,6 +431,10 @@ RSpec.describe Account do
       expect(subject.match('@alice@example.com')[1]).to eq 'alice@example.com'
     end
 
+    it 'matches full usernames with unicode domain names' do
+      expect(subject.match('@alice@🌈🌈🌈.st')[1]).to eq 'alice@🌈🌈🌈.st'
+    end
+
     it 'matches full usernames with a dot at the end' do
       expect(subject.match('Hello @alice@example.com.')[1]).to eq 'alice@example.com'
     end