diff --git a/CHANGELOG.md b/CHANGELOG.md
index fdf3d4d7146..361730366ac 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@
 - Fixed a bug where the “All” checkbox label was getting HTML-encoded when using `Craft.ui.createCheckboxSelect()`.
 - Fixed a bug where week day and month names were being translated based on the current formatting locale, rather than the current language. ([#7312](https://github.com/craftcms/cms/issues/7312))
 - Fixed a bug where `craft\elements\Asset::getSrcset()` could return the wrong value if the asset had a named transform set on it. ([#7352](https://github.com/craftcms/cms/issues/7352))
+- Fixed a bug where user registration forms could get a “Username cannot be blank” error even if the `useEmailAsUsername` config setting was enabled. ([#7357](https://github.com/craftcms/cms/issues/7357))
 
 ## 3.5.17.1 - 2020-12-17
 
diff --git a/src/elements/User.php b/src/elements/User.php
index adcad91d2b5..28e19ef39d1 100644
--- a/src/elements/User.php
+++ b/src/elements/User.php
@@ -698,10 +698,14 @@ protected function defineRules(): array
         $rules[] = [['email', 'unverifiedEmail'], 'email', 'enableIDN' => $enableIdn];
         $rules[] = [['email', 'password', 'unverifiedEmail'], 'string', 'max' => 255];
         $rules[] = [['username', 'firstName', 'lastName', 'verificationCode'], 'string', 'max' => 100];
-        $rules[] = [['username', 'email'], 'required'];
-        $rules[] = [['username'], UsernameValidator::class];
+        $rules[] = [['email'], 'required'];
         $rules[] = [['lastLoginAttemptIp'], 'string', 'max' => 45];
 
+        if (!Craft::$app->getConfig()->getGeneral()->useEmailAsUsername) {
+            $rules[] = [['username'], 'required'];
+            $rules[] = [['username'], UsernameValidator::class];
+        }
+
         if (Craft::$app->getIsInstalled()) {
             $rules[] = [
                 ['username', 'email'],