<p>Sexual and gendered harassment is prevalent among adolescents and associated with health outcomes. Evidence from randomized controlled trials of school-based interventions addressing such harassment outside dating contexts among students under 16 years is limited. This study evaluated the effectiveness of the Stop Sexual Harassment (SSH) program in secondary schools. The cluster-randomized controlled superiority trial was conducted in 41 Norwegian lower secondary schools. Participants were 2236 students aged 13–15 years. Schools were randomized 1:1 to SSH or a delayed-access control using centralized, computer-generated, pair-matched randomization at the school level. The intervention comprised a digital teacher-training course and eight structured classroom lessons delivered during regular school hours; control schools continued their usual practice and were offered SSH after follow-up. Primary outcomes were self-reported sexual and gendered harassment victimization and perpetration measured with eight prespecified subscales at baseline and 2 months after baseline. Analyses followed the intention-to-treat principle using three-level mixed-effects models with baseline adjustment. Exploratory per-protocol analyses examined students with sufficient intervention exposure. Twenty schools (1046 students) were allocated to SSH and 21 schools (1190 students) to control. All randomized students were included in the primary analyses. More than half of the participants had missing outcome data; this was addressed using multiple imputation. No statistically significant between-group differences were observed for any primary outcome at post-intervention; effect sizes were small and close to zero. In conclusion, this cluster-randomized trial found no evidence that SSH reduced sexual and gendered harassment compared with a delayed-access control condition.</p>