This research investigates the link between rivalry and unethical behavior. We propose that people will be more willing and likely to engage in unethical behavior when competing against their rivals than when competing against non-rival competitors. Further, we argue that rivalry may act as a mindset such that mere exposure to one’s rivals can be enough to incite unethical behavior even in domains unrelated to that rivalrous relationship. Across a series of experiments and an archival study, we found that rivalry was associated with over-reporting of performance, deception, and unsportsmanlike behavior. Further, we observed that merely thinking about a rival was enough to increase unethicality. These findings highlight the importance of rivalry as a widespread, powerful, yet largely unstudied phenomenon with significant organizational implications. Further, the results help to inform when and why unethical behavior occurs within organizations, and demonstrate that the nature of competition is dependent upon actors’ relationships and prior interactions.