A federated realm discovery system within a federation determines a “home” realm associated with a portion of the user's credentials before the user's secret information (such as a password) is passed to a non-home realm. A login user interface accepts a user identifier and, based on the user identifier, can use various methods to identify an account authority service within the federation that can authenticate the user. In one method, a realm list of the user device can be used to direct the login to the appropriate home realm of the user. In another method, an account authority service in a non-home realm can look up the user's home realm and provide realm information directing the user device to login at the home realm.