diff --git a/app/users/directives/username-validator.js b/app/users/directives/username-validator.js
new file mode 100644
index 0000000..d8c5848
--- /dev/null
+++ b/app/users/directives/username-validator.js
@@ -0,0 +1,38 @@
+window.angular && (function(angular) {
+  'use strict';
+
+  /**
+   * Username validator
+   *
+   * Checks if entered username is a duplicate
+   * Provide existingUsernames scope that should be an array of
+   * existing usernames
+   *
+   * <input username-validator  existing-usernames="[]"/>
+   *
+   */
+  angular.module('app.users').directive('usernameValidator', function() {
+    return {
+      restrict: 'A', require: 'ngModel', scope: {existingUsernames: '='},
+          link: function(scope, element, attrs, controller) {
+            if (scope.existingUsernames === undefined) {
+              return;
+            }
+            controller.$validators.duplicateUsername =
+                (modelValue, viewValue) => {
+                  const enteredUsername = modelValue || viewValue;
+                  const matchedExisting = scope.existingUsernames.find(
+                      (username) => username === enteredUsername);
+                  if (matchedExisting) {
+                    return false;
+                  } else {
+                    return true;
+                  }
+                };
+            element.on('blur', () => {
+              controller.$validate();
+            });
+          }
+    }
+  });
+})(window.angular);