Systems and methods for authentication of users using online activity are provided. An authentication system may build a user profile in a database by associating online activity data with a user in the user profile, and further by processing the online activity data to generate user interest data and associating it with the user. The authentication system may receive a payment request for a purchase of an item using a payment account of the user. The authentication system may determine if the purchase of the item by the user is consistent with the user's online activity data and/or user interest data. If consistent, the authentication system may authenticate the user and process the payment request by approving the payment request.