ABSTRACT In the ongoing debate regarding the impact of puppy love on adolescents’ personal development, the existing literature has paid insufficient attention to the externalities of students experiencing puppy love in classrooms. By using a nationally representative survey of middle school students in China, we employed random student-classroom assignment as a quasi-experiment to investigate the causal effect of classroom peers experiencing puppy love on students’ academic performance. We found that classroom peers in puppy love had an adverse impact on students’ academic performance, which is proven to be reliable by a series of robustness checks, including instrumental variable(IV)estimation, removing extreme observations, placebo tests, and adding other control variables. Mechanism analysis indicated that this negative peer effect was caused by students’ lower learning effort, lower expectations for the future, further peer effect of student friends and teachers’ lack of responsibility and patience. Furthermore, this negative effect was more pronounced among boys, students with high family income, and medium or low cognitive ability.