Background: Employee performance, rates of illness, absenteeism, and staff turnover are all affected by employees’ mental health status. Managers are in a unique position to address workplace mental health risk factors and implement changes that have a positive effect on the mental health and wellbeing of employees. However, many managers report a lack of knowledge and confidence in promoting workplace mental health, suggesting a need for formal training. Aims: We aimed to test the efficacy of a newly developed, digital mental health training intervention for managers, featured on Unmind – a workplace wellbeing platform that can be accessed via a mobile or web app. Methods: We conducted a web-based, parallel-group, randomized controlled trial. Participants were digitally-engaged managers in desk-based roles, residing in the US or UK, who were randomized to the intervention or a no-intervention control group with a 1:1 allocation ratio. The primary outcome was manager confidence to create mentally healthy workplaces, whilst secondary outcomes included manager knowledge, behavior, mental health stigma, and compassion for self and others. Outcomes were captured at baseline (t0), post-intervention (t1) and 3-month follow-up (t2), and analyzed using mixed effects models with intention-to-treat principles. Results: 300 participants were randomized, and 91.7% were retained at t2. 55.7% of managers randomized to the training completed all sessions. All outcomes showed significantly larger improvement in the intervention group compared to control at t1 and t2, except for self-compassion at t1 and mental health stigma which was marginal at both timepoints. The between-group Hedges’ g effect size for manager confidence was 0.38 [0.15 – 0.61] at t1 (the primary endpoint) and 0.48 [0.25 – 0.71] at t2. Conclusions: These findings show that a brief, digital training intervention can improve manager knowledge and confidence, leading to positive changes in manager behavior. Given the accessibility of such interventions, this could have major implications for mitigating the human and financial impact of poor mental health in the workplace.