We studied the influence of right-wing authoritarianism (RWA) on the relationship between a multicultural ideology and attitudes about ethnic diversity and immigrants. We hypothesized that a multicultural ideology poses a threat to authoritarian individuals, which leads to a decrease in positive diversity beliefs and an increase in prejudice toward immigrants. On the basis of representative survey-data from 23 European countries, we showed that the negative relationship between RWA and positive diversity beliefs was stronger the more a country engages in multiculturalism (Study 1). In addition, in two experiments we demonstrated that RWA moderated the relationship between a video promoting multiculturalism (Study 2) or a picture showing a multicultural group (Study 3) and attitudes toward immigrants and diversity. As expected, for high-RWAs, both stimuli led to an increase in prejudice. In Study 3, perceived threat mediated the relationship between a multicultural norm and prejudice for people high in RWA.