This study investigated the educational spillover effects of migrant peers on students' academic achievement in urban China. Using the random assignment of students and educational resources to classes within a school, this study tested the causal effects of the proportion of migrant students in a class on three test scores of local and migrant students. The results show that migrant peers influenced the academic achievement of both types of students, and that students in classes with higher proportions of migrants scored lower on the tests. Local students were more strongly influenced than migrant students. Analysis of the possible mechanisms suggested that the effects were driven by teachers' adjustments to their pedagogical practices rather than peer interaction or learning environment. The effects were nonlinear, and heterogeneous regarding gender, school type, and city size. These results will help policymakers and educators understand the characteristics and sources of migrant peer effects and support effective integration policies for internal migrant students, particularly in developing countries.