ObjectiveHigh-intensity interval training (HIIT) is a safe and feasible form of exercise. The aim of this meta-analysis was to investigate the mental health effects of HIIT, in healthy populations and those with physical illnesses, and to compare the mental health effects to non-active controls and other forms of exercise.DesignRandom effects meta-analyses were undertaken for randomised controlled trials (RCTs) comparing HIIT with non-active and/or active (exercise) control conditions for the following coprimary outcomes: mental well-being, symptoms of depression, anxiety and psychological stress. Positive and negative affect, distress and sleep outcomes were summarised narratively.Data sourcesMedline, PsycINFO, Embase and CENTRAL databases were searched from inception to 7 July 2020.Eligibility criteria for selecting studiesRCTs that investigated HIIT in healthy populations and/or those with physical illnesses and reported change in mental well-being, depression, anxiety, psychological stress, positive/negative affect, distress and/or sleep quality.ResultsFifty-eight RCTs were retrieved. HIIT led to moderate improvements in mental well-being (standardised mean difference (SMD): 0.418; 95% CI: 0.135 to 0.701; n=12 studies), depression severity (SMD: –0.496; 95% CI: −0.973 to −0.020; n=10) and perceived stress (SMD: −0.474; 95% CI: −0.796 to −0.152; n=4) compared with non-active controls, and small improvements in mental well-being compared with active controls (SMD:0.229; 95% CI: 0.054 to 0.403; n=12). There was a suggestion that HIIT may improve sleep and psychological distress compared with non-active controls: however, these findings were based on a small number of RCTs.ConclusionThese findings support the use of HIIT for mental health in the general population.Level of evidenceThe quality of evidence was moderate-to-high according to the Grading of Recommendations Assessment, Development and Evaluation) criteria.PROSPERO registration numberCRD42020182643