Are humans ever truly altruistic? Or are all actions, however noble, ultimately motivated by self-interest? Scientists and philosophers have long grappled with this question, but few have considered laypeople’s beliefs about the nature of prosocial motives. Here we examine these beliefs and their social correlates. In line with prior work, we find that people tend to believe humans can be, and frequently are, altruistically motivated. Moreover, people who more strongly believe in altruistic motivation act more prosocially themselves—for instance, sacrificing relatively high levels of money and time to help others—a relationship that holds even when controlling for trait empathy. People who believe in altruism also judge other prosocial agents to be more genuinely kind, especially when agents’ motives are ambiguous. Together, this work suggests that believing in altruism predicts the extent to which people both see altruism and act altruistically, possibly reflecting the self-fulfilling nature of such lay theories.