The present research examines the implications of psychological essentialism for attitudes toward same-sex marriage (SSM), a hotly contested policy issue. Based on the literature on psychological essentialism, we tested the novel proposition that negative SSM attitudes are the result of essentialist thinking about the institution of marriage itself, the idea that marriage is universal, unique as a human union, invariant, and not the result of human agency. Two studies (n = 351 and n = 117) confirm these predictions, but also demonstrate the essentialist conceptions of marriage are more potent predictors of SSM attitudes than essentialist conceptions of homosexuality. Additional analyses indicated that essentialist conceptions of homosexuality and marriage did mediate the effects of religiosity and political orientation on SSM attitudes. The discussion focuses on the implications for the ongoing policy debate.