When compared with the general population, people living with severe mental illness (SMI) are 1·8 times more likely to have obesity while in adult mental health secure units, rates of obesity are 20 % higher than the general population. In England, there are currently 490 000 people living with SMI. The aim of this systematic review was to collate and synthesise the available quantitative and qualitative evidence on a broad range of weight management interventions for adults living with SMI and overweight or obesity. Primary outcomes were reductions in BMI and body weight. Following sifting, eighteen papers were included in the final review, which detailed the results of nineteen different interventions; however, there was a lack of qualitative evidence. Pooled results for three studies (MD − 3·49, 95 % CI − 6·85, −0·13,P= 0·04) indicated a small effect in terms of body weight reduction but no effect on BMI for four studies (MD − 0·42, 95 % CI − 1·27, 0·44,P= 0·34). Key recommendations for future research included integration of qualitative methodology into experimental study design, a review of outcome measures and for study authors to follow standardised guidelines for reporting to facilitate complete and transparent reporting.