diff --git a/cli/onionshare_cli/resources/static/js/chat.js b/cli/onionshare_cli/resources/static/js/chat.js
index a221106c..21f00ca6 100644
--- a/cli/onionshare_cli/resources/static/js/chat.js
+++ b/cli/onionshare_cli/resources/static/js/chat.js
@@ -82,7 +82,7 @@ var emitMessage = function (socket) {
 
 var updateUsername = function (socket) {
   var username = $('#username').val();
-  if (!checkUsernameExists(username) && !checkUsernameTooLong(username)) {
+  if (!checkUsernameExists(username) && !checkUsernameTooLong(username) && !checkUsernameAscii(username)) {
     $.ajax({
       method: 'POST',
       url: `http://${document.domain}:${location.port}/update-session-username`,
@@ -117,6 +117,16 @@ var createUserListHTML = function (connected_users, current_user) {
   return userListHTML;
 }
 
+var checkUsernameAscii = function (username) {
+  // ASCII characters have code points in the range U+0000-U+007F.
+  $('#username-error').text('');
+  if (!/^[\u0000-\u007f]*$/.test(username)) {
+    $('#username-error').text('Non-ASCII usernames are not supported.');
+    return true;
+  }
+  return false;
+}
+
 var checkUsernameExists = function (username) {
   $('#username-error').text('');
   var userMatches = $('#user-list li').filter(function () {
diff --git a/cli/onionshare_cli/web/chat_mode.py b/cli/onionshare_cli/web/chat_mode.py
index 3eda2867..f6aead45 100644
--- a/cli/onionshare_cli/web/chat_mode.py
+++ b/cli/onionshare_cli/web/chat_mode.py
@@ -51,6 +51,7 @@ class ChatModeWeb:
         username = username.strip()
         return (
             username
+            and username.isascii()
             and username not in self.connected_users
             and len(username) < 128
         )