South Korea, a relatively collectivistic and homogeneous country with heightened cultural tightness, is believed to have particularly high levels of stigma toward autistic individuals, who sometimes engage in behaviors that diverge from social norms. This study investigated cross-cultural differences in autism stigma (assessed with a Social Distance Scale) in the United States and South Korea. Two-hundred and seventy-six American and 494 Korean participants who completed an online survey were included in the analysis. We conducted a multiple regression predicting autism stigma with variables that were correlated with stigma. Koreans reported greater autism stigma than Americans. Greater vertical individualism, lesser horizontal collectivism, less accurate autism knowledge, less pleasant and frequent previous contact with autism, concerns about the marriageability of family members, and higher cultural tightness predicted greater stigma. Cultural tightness, or an emphasis on social norms, which was heightened among Korean participants, contributed to greater autism stigma in South Korea. Findings highlight the need to increase autism knowledge and foster pleasant and frequent contact with autistic individuals, especially for those who accept inequality as a part of human interactions in both South Korea and the United States. Moreover, interventions that help Koreans understand the relativeness of social appropriateness may reduce autism stigma in South Korea.