diff --git a/app/helpers/accounts_helper.rb b/app/helpers/accounts_helper.rb
index 99815be7b..53939adfc 100644
--- a/app/helpers/accounts_helper.rb
+++ b/app/helpers/accounts_helper.rb
@@ -13,7 +13,7 @@ module AccountsHelper
     if account.local?
       "@#{account.acct}@#{Rails.configuration.x.local_domain}"
     else
-      "@#{account.acct}"
+      "@#{account.pretty_acct}"
     end
   end
 
diff --git a/app/models/account.rb b/app/models/account.rb
index 884332e5a..feaf273c1 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -50,7 +50,7 @@
 
 class Account < ApplicationRecord
   USERNAME_RE = /[a-z0-9_]+([a-z0-9_\.-]+[a-z0-9_]+)?/i
-  MENTION_RE  = /(?<=^|[^\/[:word:]])@((#{USERNAME_RE})(?:@[a-z0-9\.\-]+[a-z0-9]+)?)/i
+  MENTION_RE  = /(?<=^|[^\/[:word:]])@((#{USERNAME_RE})(?:@[[:word:]\.\-]+[a-z0-9]+)?)/i
 
   include AccountAssociations
   include AccountAvatar
@@ -164,6 +164,10 @@ class Account < ApplicationRecord
     local? ? username : "#{username}@#{domain}"
   end
 
+  def pretty_acct
+    local? ? username : "#{username}@#{Addressable::IDNA.to_unicode(domain)}"
+  end
+
   def local_username_and_domain
     "#{username}@#{Rails.configuration.x.local_domain}"
   end
diff --git a/app/serializers/rest/account_serializer.rb b/app/serializers/rest/account_serializer.rb
index 5fec75673..fd361a34c 100644
--- a/app/serializers/rest/account_serializer.rb
+++ b/app/serializers/rest/account_serializer.rb
@@ -24,6 +24,10 @@ class REST::AccountSerializer < ActiveModel::Serializer
     object.id.to_s
   end
 
+  def acct
+    object.pretty_acct
+  end
+
   def note
     Formatter.instance.simplified_format(object)
   end
diff --git a/app/services/process_mentions_service.rb b/app/services/process_mentions_service.rb
index 2f7a9e985..b2d868165 100644
--- a/app/services/process_mentions_service.rb
+++ b/app/services/process_mentions_service.rb
@@ -14,7 +14,16 @@ class ProcessMentionsService < BaseService
     mentions = []
 
     status.text = status.text.gsub(Account::MENTION_RE) do |match|
-      username, domain  = Regexp.last_match(1).split('@')
+      username, domain = Regexp.last_match(1).split('@')
+
+      domain = begin
+        if TagManager.instance.local_domain?(domain)
+          nil
+        else
+          TagManager.instance.normalize_domain(domain)
+        end
+      end
+
       mentioned_account = Account.find_remote(username, domain)
 
       if mention_undeliverable?(mentioned_account)
diff --git a/spec/services/process_mentions_service_spec.rb b/spec/services/process_mentions_service_spec.rb
index b1abd79b0..c30de8eeb 100644
--- a/spec/services/process_mentions_service_spec.rb
+++ b/spec/services/process_mentions_service_spec.rb
@@ -5,11 +5,11 @@ RSpec.describe ProcessMentionsService, type: :service do
   let(:visibility) { :public }
   let(:status)     { Fabricate(:status, account: account, text: "Hello @#{remote_user.acct}", visibility: visibility) }
 
+  subject { ProcessMentionsService.new }
+
   context 'OStatus with public toot' do
     let(:remote_user) { Fabricate(:account, username: 'remote_user', protocol: :ostatus, domain: 'example.com', salmon_url: 'http://salmon.example.com') }
 
-    subject { ProcessMentionsService.new }
-
     before do
       stub_request(:post, remote_user.salmon_url)
       subject.call(status)
@@ -24,8 +24,6 @@ RSpec.describe ProcessMentionsService, type: :service do
     let(:visibility)  { :private }
     let(:remote_user) { Fabricate(:account, username: 'remote_user', protocol: :ostatus, domain: 'example.com', salmon_url: 'http://salmon.example.com') }
 
-    subject { ProcessMentionsService.new }
-
     before do
       stub_request(:post, remote_user.salmon_url)
       subject.call(status)
@@ -41,29 +39,45 @@ RSpec.describe ProcessMentionsService, type: :service do
   end
 
   context 'ActivityPub' do
-    let(:remote_user) { Fabricate(:account, username: 'remote_user', protocol: :activitypub, domain: 'example.com', inbox_url: 'http://example.com/inbox') }
+    context do
+      let(:remote_user) { Fabricate(:account, username: 'remote_user', protocol: :activitypub, domain: 'example.com', inbox_url: 'http://example.com/inbox') }
 
-    subject { ProcessMentionsService.new }
+      before do
+        stub_request(:post, remote_user.inbox_url)
+        subject.call(status)
+      end
 
-    before do
-      stub_request(:post, remote_user.inbox_url)
-      subject.call(status)
+      it 'creates a mention' do
+        expect(remote_user.mentions.where(status: status).count).to eq 1
+      end
+
+      it 'sends activity to the inbox' do
+        expect(a_request(:post, remote_user.inbox_url)).to have_been_made.once
+      end
     end
 
-    it 'creates a mention' do
-      expect(remote_user.mentions.where(status: status).count).to eq 1
-    end
+    context 'with an IDN domain' do
+      let(:remote_user) { Fabricate(:account, username: 'sneak', protocol: :activitypub, domain: 'xn--hresiar-mxa.ch', inbox_url: 'http://example.com/inbox') }
+      let(:status) { Fabricate(:status, account: account, text: "Hello @sneak@hæresiar.ch") }
 
-    it 'sends activity to the inbox' do
-      expect(a_request(:post, remote_user.inbox_url)).to have_been_made.once
+      before do
+        stub_request(:post, remote_user.inbox_url)
+        subject.call(status)
+      end
+
+      it 'creates a mention' do
+        expect(remote_user.mentions.where(status: status).count).to eq 1
+      end
+
+      it 'sends activity to the inbox' do
+        expect(a_request(:post, remote_user.inbox_url)).to have_been_made.once
+      end
     end
   end
 
   context 'Temporarily-unreachable ActivityPub user' do
     let(:remote_user) { Fabricate(:account, username: 'remote_user', protocol: :activitypub, domain: 'example.com', inbox_url: 'http://example.com/inbox', last_webfingered_at: nil) }
 
-    subject { ProcessMentionsService.new }
-
     before do
       stub_request(:get, "https://example.com/.well-known/host-meta").to_return(status: 404)
       stub_request(:get, "https://example.com/.well-known/webfinger?resource=acct:remote_user@example.com").to_return(status: 500)