Background and Aims: This study evaluated whether a preventive intervention program for problem gambling would increase managers inclination to act when concerned about gambling in the workplace. Design: Cluster randomized controlled trial. Ten workplaces were randomized to either intervention or control condition. Participants: At the 12-month endpoint, there were n = 136 managers and n = 1594 subordinates in the intervention group, and n = 137 managers and n = 1150 subordinates in the waitlist group. Intervention: The intervention consisted of (1) six hours of skill-development training for managers regarding gambling, problem gambling, gaming, and harmful use of psychoactive drugs, and (2) six to eight hours of assistance in developing or improving workplace gambling policy. Measurements: The primary outcome was the managers self-rated (on a 1 to 10 scale) inclination to act when concerned about an employees problem gambling 12 months after baseline. Findings: The between -group difference in the managers inclination for the full intervention group (M = 8) and the control group (M = 7.4) was not significant at the 12-month follow-up, but it was when only including managers who attended the skill development training (M = 8.2), d = 0.31, p = .04. Conclusion: A workplace prevention program aimed to increase managers inclination to act when they are concerned regarding an employees gambling resulted in statistically significant changes for those who attended training, but not for the whole intervention group when non-attendees were included.