This cross-cultural study examined Koreans' and Americans' explicit stigma and implicit biases toward autism to examine potential mechanisms underlying recent evidence for heightened explicit autism stigma in South Korea relative to the United States. This evidence is somewhat at odds with other evidence that individuals living in collectivistic cultures such as South Korea may be more prone to present themselves favorably than those living in relatively individualistic cultures such as the United States. A total 224 American and 536 Korean non-autistic adult participants completed an online survey. Implicit biases were measured using the implicit association test. Koreans reported greater explicit stigma and exhibited more implicit biases toward autism than Americans. Explicit stigma was not correlated with implicit biases in either country. Less autism knowledge and pleasant contact with autistic people predicted greater explicit stigma among both Koreans and Americans. Less frequent contact and heightened assimilation ideology toward ethnic minorities predicted greater stigma only among Koreans. The variance in implicit biases explained by predictors was small, emphasizing the need for follow-up research investigating predictors of implicit biases. Informing Koreans about the shortcomings of assimilationist approaches and fostering an appreciation of the plurality of cultures may reduce stigma toward autistic individuals in South Korea.