diff --git a/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php b/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
index c389191..e6bcc3e 100644
--- a/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
+++ b/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
@@ -29,7 +29,7 @@ public function validate($items, Constraint $constraint) {
     if (strpos($name, '  ') !== FALSE) {
       $this->context->addViolation($constraint->multipleSpacesMessage);
     }
-    if (preg_match('/[^\x{80}-\x{F7} a-z0-9@_.\'-]/i', $name)
+    if (preg_match('/[^\x{80}-\x{F7} a-z0-9@+_.\'-]/i', $name)
       || preg_match(
         '/[\x{80}-\x{A0}' .       // Non-printable ISO-8859-1 + NBSP
         '\x{AD}' .                // Soft-hyphen
diff --git a/core/modules/user/src/Tests/UserValidationTest.php b/core/modules/user/src/Tests/UserValidationTest.php
index ffa8d95..4c545ff 100644
--- a/core/modules/user/src/Tests/UserValidationTest.php
+++ b/core/modules/user/src/Tests/UserValidationTest.php
@@ -48,6 +48,7 @@ function testUsernames() {
       'foo@example.com'        => array('Valid username', 'assertNull'),
       'foo@-example.com'       => array('Valid username', 'assertNull'), // invalid domains are allowed in usernames
       'þòøÇߪř€'               => array('Valid username', 'assertNull'),
+      'foo+bar'                => array('Valid username', 'assertNull'), // '+' symbol is allowed
       'ᚠᛇᚻ᛫ᛒᛦᚦ'                => array('Valid UTF8 username', 'assertNull'), // runes
       ' foo'                   => array('Invalid username that starts with a space', 'assertNotNull'),
       'foo '                   => array('Invalid username that ends with a space', 'assertNotNull'),