diff --git a/CHANGELOG_DEV.md b/CHANGELOG_DEV.md
index 2f4c84c309..f5eb708c93 100644
--- a/CHANGELOG_DEV.md
+++ b/CHANGELOG_DEV.md
@@ -42,3 +42,4 @@
 - Enh #4671: Implemented live poll result sharing by BroadCastChannel
 - Enh #4829: Renamed account "Security" page and make it optional
 - Enh #4818: Added database related checks (Driver, Collation, Table Engine)
+- Fix #4831: Validate username by pattern only on change or for new users
diff --git a/protected/humhub/modules/user/models/User.php b/protected/humhub/modules/user/models/User.php
index dc8137a103..4045a00cbd 100644
--- a/protected/humhub/modules/user/models/User.php
+++ b/protected/humhub/modules/user/models/User.php
@@ -150,7 +150,9 @@ class User extends ContentContainerActiveRecord implements IdentityInterface, Se
             [['username'], 'unique'],
             [['username'], 'string', 'max' => $userModule->maximumUsernameLength, 'min' => $userModule->minimumUsernameLength],
             // Client validation is disable due to invalid client pattern validation
-            [['username'], 'match', 'pattern' => $userModule->validUsernameRegexp, 'message' => Yii::t('UserModule.base', 'Username contains invalid characters.'), 'enableClientValidation' => false],
+            [['username'], 'match', 'pattern' => $userModule->validUsernameRegexp, 'message' => Yii::t('UserModule.base', 'Username contains invalid characters.'), 'enableClientValidation' => false, 'when' => function ($model, $attribute) {
+                return $model->getAttribute($attribute) !== $model->getOldAttribute($attribute);
+            }],
             [['status', 'created_by', 'updated_by', 'visibility'], 'integer'],
             [['tags'], 'string'],
             [['guid'], 'string', 'max' => 45],