This meta-analytic review responds to promises in the research literature and public domain about the benefits of workplace mindfulness training. It synthesizes randomized controlled trial evidence from workplace-delivered training for changes in mindfulness, stress, mental health, well-being, and work performance outcomes. Going beyond extant reviews, this article explores the influence of variability in workforce and intervention characteristics for reducing perceived stress. Meta-effect estimates (Hedge's g) were computed using data from 23 studies. Results indicate beneficial effects following training for mindfulness (g = 0.45, p < .001) and stress (g = 0.56, p < .001), anxiety (g = 0.62, p < .001) and psychological distress (g = 0.69, p < .001), and for well-being (g = 0.46, p = .002) and sleep (g = 0.26, p = .003). No conclusions could be drawn from pooled data for burnout due to ambivalence in results, for depression due to publication bias, or for work performance due to insufficient data. The potential for integrating the construct of mindfulness within job demands-resources, coping, and prevention theories of work stress is considered in relation to the results. Limitations to study designs and reporting are addressed, and recommendations to advance research in this field are made. (PsycINFO Database Record (c) 2019 APA, all rights reserved).