There is a mortality gap of 15 to 20 years for people with severe mental illness (SMI - psychotic spectrum, bipolar, major depressive disorders). Modifiable risk factors include inactivity and low cardiorespiratory fitness (CRF). Exercise can improve mental and physical outcomes; optimal type and intensity of exercise for people with SMI has yet to be determined. High Intensity Interval training (HIIT) is an exercise with distinct cardio-metabolic advantages in other disease populations compared to traditional moderate intensity continuous training (MCT). We investigated the feasibility and efficacy of HIIT for people with SMI. Major electronic databases were searched, identifying HIIT studies for adults experiencing SMI.Data on feasibility, safety, study design, sample characteristics, and physical and psychological outcomes were extracted and systematically reviewed. Meta-analyses were conducted within group, pre and post HIIT interventions, and between group, to compare HIIT with control conditions. Nine articles were identified including three pre/post studies, one non randomised and five randomised trials, (366 participants, 45.1% female). HIIT appears as feasible as MCT, with few safety concerns. Following HIIT, there was a moderate improvement in CRF and depression. There was no difference between HIIT and MCT for adherence or CRF.HIIT improved depression more than MCT.