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. Lay abstract Misunderstandings about autism may be more common in South Korea than the United States. Koreans often have clear ideas about how people should act. Another way of saying this is that Korea has a tight culture. Americans are looser, meaning people are freer to act as they like. Autistic people often do not act as people expect them to. This makes autistic people stand out. Autistic people may stand out more in tight cultures like South Korea. We studied how people in South Korea and the United States feel about autism. We wanted to see why Korean people might reject autistic people more than people in the United States do. American and Korean people did online surveys. Koreans said they did not want to get close to autistic people more than Americans did. People who understood autism and had met and liked autistic people wanted to get closer to autistic people. We were surprised to learn that Americans said having an autistic brother or sister makes it harder to find a romantic partner more than Korean people did. People who believed that autism makes it harder for family members to find love did not want to get very close to autistic people. Koreans said people should act as expected more than Americans did. People who believed that acting as expected was important did not want to get very close to autistic people. Teaching people that there are many ways of being a good person may help them understand and appreciate autistic people.