In recent years, Americans have become more affectively polarized: That is, ordinary Democrats and Republicans increasingly dislike and distrust members of the opposing party. Such polarization is normatively troubling, as it exacerbates gridlock and dissensus in Washington. Given these negative consequences, I investigate whether it is possible to ameliorate this partisan discord. Building on the Common Ingroup Identity Model from social psychology, I show that when subjects’ sense of American national identity is heightened, they come to see members of the opposing party as fellow Americans rather than rival partisans. As a result, they like the opposing party more, thereby reducing affective polarization. Using several original experiments, as well as a natural experiment surrounding the July 4th holiday and the 2008 summer Olympics, I find strong support for my argument. I conclude by discussing the implications of these findings for efforts to reduce polarization more generally.