The minimal group effect (MGE) is one of the most robust psychological findings in studies of intergroup conflict, yet there is little evidence comparing its magnitude across cultures. Recent evidence suggests that the MGE is due in part to a projection of one’s own perceived characteristics onto the novel in-group. Because of cultural variability in self-enhancement motivations, we thus expected that those from East Asian cultures would exhibit a diminished MGE relative to Westerners. A large and diverse sample of Japanese and American participants completed a traditional minimal group study. American participants were more likely to show an in-group bias in group identification, perceived group intelligence, perceived group personality traits, and resource allocation. Furthermore, these cultural differences were partially mediated by self-esteem. We discuss the implication of these findings for theories of intergroup conflict and suggest multiple directions for future cross-cultural research on the MGE.