Peer victimization represents a pervasive problem, particularly for students in middle school. Although curriculum-based prevention programs have generated small to moderate effects on victimization, these effects tend to weaken beginning with the transition to middle school. In this study, we evaluated cooperative learning (CL) as a mechanism to prevent victimization, and evaluated reciprocated friendships as a mediator of these effects. Using four waves of data from a cluster randomized trial of CL (7 intervention and 8 control middle schools; N = 1,890 students, 47.1% female, 75.2% White), we found that CL significantly reduced victimization after two years, and these effects were mediated by growth in the ratio of reciprocated friendship in the first year. We conclude that CL can reduce victimization by providing a means for students to engage in extended social interactions with a wider range of peers and thus creating opportunities for students to forge stronger (i.e., reciprocated) friendships.