This study examined the link between classroom ethnic diversity, cross-ethnic friendships, and children's theory of mind. In total, 730 children in the United Kingdom (54.7% girls, 51.5% White) aged 8 to 13 years completed measures of theory of mind in 2019/2020. Controlling for verbal ability, executive function, peer social preference, and teacher-reported demographic characteristics, greater classroom ethnic diversity provided opportunities for cross-ethnic friendships, and children with cross-ethnic friendships performed better than peers without cross-ethnic friendships on theory of mind. These results extend accounts of intergroup contact by using direct assessments of children's theory of mind and advance social accounts of theory of mind by demonstrating how experiences outside the family are linked with theory of mind.