Background: Despite the critical role of educators as gatekeepers for school mental health services, they receive limited training to support student mental health. We report findings from a trial of an online mental health role-play simulation for elementary school teachers on changes in attitudes and self-reported helping behaviors for students experiencing psychological distress. Methods: We randomly assigned 18,896 elementary school teachers to wait-list control or intervention conditions in which they received the 45- to 90-minute online role-play simulation. We administered a version of the validated Gatekeeper Behavior Scale at baseline and postintervention, which measures attitudinal dimensions shown to predict teacher helping behavior change. Self-reported helping behaviors were collected at baseline and 3-month follow-up. Outcomes were compared between the intervention follow-up and control group baseline measures. Results: The intervention group posttraining scores were significantly higher (p < 0.001) than the control group for all the preparedness, likelihood, and self-efficacy Gatekeeper Behavior subscales. All 5 helping behaviors were significantly higher among the intervention group at follow-up compared to the control group at baseline. Conclusions: We found that a brief online role-play simulation was an effective strategy for improving teacher attitudes and behaviors needed to perform a positive mental health gatekeeper role in schools.