Users of a social networking system can tag other users in content items that are provided outside of the social networking system, such as pictures that are viewable on a website that is external to the social networking system. To enable a user to tag another user in a content item, an external system provides a user interface that enables the user to tag another user, optionally define a portion of the content item in which the tagged user appears, and provide an identity of the tagged user. The external system communicates with the social networking system to provide the tagging information to the social networking system. The tagging information is received by the social networking system, which imports the content item and displays the content item with the tagging information to other users in the social networking system.