Self-regulated study significantly affects human capital accumulation and cognitive and noncognitive development; thus, developing such abilities is important for adolescents. The current literature offers little evidence on how peers affect students’ self-regulated study. Through examining a random assignment of peers across classrooms within schools in South Korea, this article investigates the effects of peers’ self-regulated study on individual students’ self-regulated study. We apply a school fixed effect coupled with an instrumental variable approach in a regression analysis to identify the causal relations. Results show that students increase their self-regulated study time in response to their peers’ self-regulated study time. The effects are statistically significant for the subsamples of coeducational schools, urban schools, public schools, high schools, and lower academic achievement. Our findings confirm the presence of peer effects on self-regulated study and suggest policy implications regarding school management, such as classroom organization, to enhance the educational environment.