We tested whether greater identification as a user of a substance (i.e., perceived general associations between oneself and use of the substance, the group of people who use the substance, and/or the substance itself) results from a theorized source-perceived fit between one's own identity and one's perception of the identity of being a user of the substance. We also tested whether this effect is stronger the more the group of people who use the substance is perceived as an entity and the more the prototypical user of the substance is perceived as having a favorable identity. We conducted two correlational and two (preregistered) experimental investigations of drinker identity and marijuana user identity. In all studies, substance user identity was positively associated with substance user-self fit perception. In both correlational studies, this relationship was stronger the more the group of people who use the substance was perceived as an entity.In the drinker identity correlational study, this relationship was also stronger the more it was perceived that the prototypical drinker has a favorable identity. A substance user-self fit perception could be addressed to reduce identification as a user of the substance in efforts to prevent hazardous use. Public Significance StatementAmong individuals who use a substance (e.g., consume alcohol), those who believe that doing so is part of their identity are prone to hazardous use. We demonstrated that individuals who use a substance have a substance user identity in part because they perceive a fit between their identity and their perception of the identity of being a user of the substance. This perception of identity fit could be addressed to reduce identification as a user of the substance in order to prevent hazardous use.