This paper presents evidence of gender peer effects in high schools in India using new administrative data. Identification of gender peer effects is achieved by exploiting variation induced by idiosyncratic changes in gender composition across cohorts within schools, in addition to controlling for past scores. The proportion of female classmates in a student’s cohort has a sizeable positive effect on the test scores of both male and female students. We find that peer effects vary non-linearly with the proportion of female students. Finally, we provide suggestive evidence on plausible mechanisms. We show that achievement spillovers are not the main driver of positive gender peer effects. Using a supplemental dataset, we show that a greater proportion of female students leads to an improved classroom environment in the context of Indian schools.