This study finds that the online “fake news” game, Bad News, can confer psychological resistance against common online misinformation strategies across different cultures. The intervention draws on the theory of psychological inoculation: analogous to the process of medical immunization, we find that “prebunking,” or preemptively warning and exposing people to weakened doses of misinformation, can help cultivate “mental antibodies” against fake news. We conclude that social impact games rooted in basic insights from social psychology can boost immunity against misinformation across a variety of cultural, linguistic, and political settings.