Objective: The purposes of this study were to investigate the influence of social norms on intentions to intervene in dating violence situations and, based on the Theory of Planned Behavior, to examine whether these effects were mediated by attitudes, perceived behavioral control/self-efficacy, and subjective social norms. Method: College student participants were exposed to independent manipulations of descriptive norms (i.e., whether helping victims of dating violence is common) and injunctive norms (i.e., whether helping victims of dating violence is approved of) and then completed measures of attitudes toward helping, perceived behavioral control/self-efficacy regarding helping, subjective social norms regarding helping, and intentions to help. Results: Elevating descriptive norms caused greater helping intentions, and this effect was mediated by more positive attitudes, perceived behavioral control/efficacy, and subjective norms regarding helping. Conclusions: These results represent the first causal evidence suggesting that descriptive social norms motivate people to help victims of dating violence. Furthermore, results suggest that they do so by causing people to evaluate helping behaviors more positively, believe that similar others help, and feel more confident in their ability to help. These findings suggest that the Theory of Planned Behavior is a useful framework for understanding social influence in the context of dating violence, and that dating violence interventions may benefit from manipulating descriptive social norms. (PsycINFO Database Record (c) 2017 APA, all rights reserved)