Importance Adolescent relationship abuse (ARA) and sexual violence (SV) reported among adolescents point to the need for prevention among middle school-age youths. Objective To test an athletic coach-delivered relationship abuse and sexual violence prevention program among middle school male athletes. Design, Setting, and Participants An unblinded cluster randomized clinical trial from spring 2015 to fall 2017 at 41 middle schools (38 clusters). The study included 973 male middle school athletes (ages 11-14 years; grades 6-8; participation rate 50%) followed up for 1 year (retention 86%). Interventions Coaching Boys Into Men (CBIM) is a prevention program that trains athletic coaches to talk to male athletes about (1) respectful relationship behaviors, (2) promoting more gender-equitable attitudes, and (3) positive bystander intervention when harmful behaviors among peers are witnessed. Main Outcomes and Measures The primary outcome was change in positive bystander behaviors (ie, intervening in peers' disrespectful or harmful behaviors); secondary outcomes were changes in recognition of what constitutes abusive behavior, intentions to intervene, gender-equitable attitudes, and reduction in recent ARA/SV perpetration (at end of sports season and 1-year follow up). Results Of the 973 participants, 530 were white (54.5%), 282 were black (29.0%), 14 were Hispanic (1.4%), and the remainder were multiracial, other race/ethnicity, or not reported. Positive bystander behaviors increased at end of sports season and at 1-year follow-up (relative risk, 1.51; 95% CI, 1.06-2.16 and 1.53; 95% CI, 1.10-2.12, respectively) as did recognition of abuse (mean risk difference, 0.14; 95% CI, 0.01-0.27 and 0.14; 95% CI, 0.00-0.28, respectively). At 1-year follow-up, among those who ever dated, athletes on teams receiving CBIM had lower odds of reporting recent ARA/SV perpetration (odds ratio, 0.24; 95% CI, 0.09-0.65). Gender attitudes and intentions to intervene did not differ between study arms. In exploratory intensity-adjusted and per protocol analyses, athletes on teams receiving CBIM were more likely to report positive bystander behaviors and to endorse equitable gender attitudes and less likely to report ARA and sexual harassment perpetration 1 year later. Conclusions and Relevance An athletic coach-delivered program for middle school male athletes is an effective strategy for reducing relationship abuse among younger adolescents. This randomized clinical trial tests an athletic coach-delivered relationship abuse and sexual violence prevention program among middle school male athletes. Questions Does an athletic coach-delivered gender violence prevention program for middle school male athletes increase positive bystander behaviors and reduce relationship abuse and sexual violence perpetration among youth athletes? Findings In this cluster randomized clinical trial including 973 male athletes, athletes in schools with sports teams receiving the Coaching Boys Into Men program had greater increases in positive bystander behaviors. In schools implementing the program, athletes who had ever dated were less likely to perpetrate abuse against a partner. Meaning This athletic coach-delivered program is one prevention strategy to consider for increasing positive bystander behaviors and reducing relationship abuse and sexual violence.