Pediatric primary care is a promising setting for reducing diversion of stimulant medications for ADHD. We tested if training pediatric primary care providers (PCPs) increased use of diversion prevention strategies with adolescents with ADHD. The study was a cluster-randomized trial in 7 pediatric primary care practices. Participants were pediatric PCPs (N = 76) at participating practices. Practices were randomized to a 1-h training in stimulant diversion prevention or treatment-as-usual. At baseline, 6 months, 12 months, and 18 months, PCPs rated how often they used four categories of strategies: patient/family education, medication management/monitoring, assessment of mental health symptoms/functioning, and assessment of risky behaviors. They completed measures of attitudes, implementation climate, knowledge/skill, and resource constraints. Generalized Estimating Equations estimated differences in outcomes by condition. Mediation analyses tested if changes in knowledge/skill mediated training effects on strategy use. PCPs in the intervention condition reported significantly greater use of patient/family education strategies at all follow-up time points. There were no differences between conditions in medication management, assessment of mental health symptoms/functioning, or assessment of risky behaviors. At 6 months, PCPs in the intervention condition reported more positive attitudes toward diversion prevention, stronger implementation climate, greater knowledge/skill, and less resource constraints. Differences in knowledge/skill persisted at 12 months and 18 months. Brief training in stimulant diversion had substantial and enduring effects on PCPs' self-reported knowledge/skill and use of patient/family education strategies to prevent diversion. Training had modest effects on attitudes, implementation climate, and resource constraints and did not change use of strategies related to medication management and assessment of mental health symptoms/functioning and risky behaviors. Changes in knowledge/skill accounted for 49% of the total effect of training on use of patient/family education strategies.