Past research finds that men negotiate more unethically than women, although many studies report comparable rates of unethical negotiation behaviors. Based on evolutionary psychology, we predict conditions under which sex differences in unethical negotiation behavior are more versus less pronounced. We theorize that greater levels of unethical behavior among men occur as a consequence of greater male intrasexual competition for mates. This suggests that more male unethical negotiation behavior should primarily emerge in situations associated with intrasexual competition. Using a two-wave survey design, Study 1 found a positive relationship between mating motivation and unethical negotiation behavior for male, but not female employees. Study 2 was a controlled experiment, replicating this effect and showing that the gender difference was most pronounced when negotiating with same-sex, attractive opponents. Study 3 used a similar experimental design and found support for another implication of evolutionary theory—that mating motivation would prompt unethical behavior in both men and women when the behavior constitutes a less severe norm violation. We discuss contributions to the literature on unethical behavior at work, negotiations, and the role of attractiveness in organizations.