This systematic literature review provides a state-of-the-art overview of the effectiveness of workplace interventions aimed at improving workers’ mental well-being. Specifically, it examines which types of interventions are effective for which aspects of mental well-being, both in the short and long term. To this end, we identify 125 randomised controlled trials comprising 147 interventions that vary in their objectives (prevention, treatment, or reintegration), targets (workers or work), methods (cognitive, meditation-based, physical, ergonomic, organisational, or multimethod) and delivery channels (offline, online or hybrid). Mental well-being outcomes are classified as biological and self-reported indicators of the three components of subjective mental well-being (evaluative, affective, and eudaimonic well-being). Overall, we find that, for preventing and treating early mental health symptoms, cognitive or meditation-based interventions targeting personal coping resources appear most promising. In contrast, successful reintegration seems to require a combination of cognitive and organisational interventions, with the latter addressing job demands and resources.