BACKGROUND: Mindfulness interventions are increasingly popular as an approach to improve mental well-being. To date, no Cochrane Review examines the effectiveness of mindfulness in medical students and junior doctors. Thus, questions remain regarding the efficacy of mindfulness interventions as a preventative mechanism in this population, which is at high risk for poor mental health.  OBJECTIVES: To assess the effects of psychological interventions with a primary focus on mindfulness on the mental well-being and academic performance of medical students and junior doctors. SEARCH METHODS: We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase and five other databases (to October 2021) and conducted grey literature searches.  SELECTION CRITERIA: We included randomised controlled trials of mindfulness that involved medical students of any year level and junior doctors in postgraduate years one, two or three. We included any psychological intervention with a primary focus on teaching the fundamentals of mindfulness as a preventative intervention. Our primary outcomes were anxiety and depression, and our secondary outcomes included stress, burnout, academic performance, suicidal ideation and quality of life.  DATA COLLECTION AND ANALYSIS: We used standard methods as recommended by Cochrane, including Cochrane's risk of bias 2 tool (RoB2).  MAIN RESULTS: We included 10 studies involving 731 participants in quantitative analysis.  Compared with waiting-list control or no intervention, mindfulness interventions did not result in a substantial difference immediately post-intervention for anxiety (standardised mean difference (SMD) 0.09, 95% CI -0.33 to 0.52; P = 0.67, I