A user identification system includes: a detection unit that detects prescribed motions of a person when the person boards or alights from a vehicle; an acquisition unit that acquires types of boarding information indicating that one of users boards or is on board the vehicle when one of the motions is detected; a memory that stores association information in which identification information of the users is associated with the types of boarding information corresponding to the users; a probability calculating unit configured to calculate a probability that each of the users boards or is on boarded the vehicle based on the types of boarding information and the association information; and an identification unit that identifies a user who boards or is on boarded the vehicle among the users based on the probability calculated by the probability calculating unit.