OBJECTIVE To explore the associations between adiposity indices, assessed at or after a diagnosis of prostate cancer, and mortality. DESIGN Systematic review and meta-analysis. DATA SOURCES PubMed and Embase, from inception to 16 November 2022. ELIGIBILITY CRITERIA FOR SELECTING STUDIES Cohort studies or randomised controlled trials of men with a diagnosis of prostate cancer that investigated the associations between adiposity (body mass index, waist and hip circumference, waist-to-hip ratio, and subcutaneous and visceral adipose tissue) after diagnosis and mortality outcomes. A modified version of the risk of bias for nutrition observational studies tool was used to assess risk of bias. RESULTS 79 studies were identified that investigated adiposity indices after a diagnosis of prostate cancer in relation to mortality. No randomised controlled trials were found. A non-linear dose-response meta-analysis indicated a J shaped association between body mass index and all-cause mortality (33910 men, 11095 deaths, 17 studies). The highest rate of all-cause mortality was found at the lowest and upper range of the distribution: 11-23% higher rate for a body mass index of 17-21and 4-43% higher rate for a body mass index of 30-40. The association between body mass index and mortality specific to prostate cancer was flat until body mass index reached 26- 27, and then increased linearly by 8-66% for a body mass index of 30-40 (33137 men, 2947 deaths, 13 studies), but the 95% confidence intervals were wide. These associations did not differ in most predefined subgroups by study design, number of deaths, anthropometric assessment, follow-up time, geographical location, prostate cancer risk group, and adjustment variables. No associations were found in meta-analyses between 10cm increases in waist circumference and all-cause mortality or mortality specific to prostate cancer, but only three studies were available. The few studies with data on change in weight, waist-to-hip ratio, and subcutaneous and visceral adipose tissue reported conflicting results. CONCLUSIONS This review suggests that patients with prostate cancer might benefit from maintaining a healthy weight and avoiding obesity. Future studies should investigate adiposity across different stages of cancer survivorship and use various parameters for distribution of adipose tissue.