Background: Beliefs about the malleability of intelligence (fixed or growth mindsets) are strongly influenced by teachers and parents. However, the social contagion of mindsets among one's classmates has not been given sufficient attention. Aims: This study aimed to examine the social contagion of mindsets among one's peers by investigating the relationship between classmates' mindsets and one's own mindset. Sample: In Study 1, 676 students nested within 19 classes were surveyed, and in Study 2, 848 students nested within 30 classes participated. Methods: Students were surveyed across two time points 7 months apart. Multilevel modelling was used. Results: The mindset of one's classmates at Time 1 predicted one's own mindset at Time 2 even after adjusting for one's own Time 1 mindset. These effects held even after controlling for demographic variables, social desirability, and achievement goals. Conclusion: The current study provided evidence for the social contagion of mindsets. Theoretical and practical implications are discussed.