Embodiments of authenticating linked accounts are presented herein. In an implementation, an authentication service provides functionality to form links between a plurality of user accounts. A client may then authenticate by providing credentials for one account in a group of linked accounts, and is permitted access to each account in the group of linked accounts based upon the linking. Thus, a single sign-in of a client to one account may permit the client to obtain services for service providers corresponding to multiple linked accounts, without an individual sign-in to each account.