Background: Previous findings regarding the correlation between body mass index (BMI) and survival in patients with prostate cancer are conflicting and may be disease-state dependent. We aimed to test whether BMI among patients with metastatic prostate cancer (mCRPC) is associated with overall survival (OS) and cancer-specific mortality (CSM). Methods: We identified 1577 patients with mCRPC from the control arms of 3 randomized trials. The role of BMI was investigated both as a continuous and categorical variable ( < 20 vs. 20-25 vs. 25-30 vs. > 30 km/m2). A BMI > 30 kg/m2 was considered obese. Analyses were adjusted for age, PSA, ECOG, # of metastasis and prior treatment. A Cox semi-proportional hazard model was used to predict OS, whereas the competing risks regression was used for predicting cancer-specific mortality (CSM). Results: The median (IQR) age for the patient population was 69 (63,74) years with a median (IQR) BMI of 28 (25-31) kg/m². Of the 1577 patients included, 655 had died during follow-up (median follow-up for survivors = 12 months). BMI emerged as a protective factor for OS both as a continuous variable (HR: 0.96; 95% CI: 0.94, 0.99; p = 0.015) and as a categorical variable (obesity: HR: 0.71, 95% CI: 0.53, 0.96; p = 0.027). The effect BMI on CSM was confirmed both as a continuous (SHR: 0.94; 95% CI: 0.91, 0.98; p = 0.002) and as a categorical variable (obesity SHR: 0.65; 95% CI: 0.45, 0.93; p = 0.018). No interaction was detected between the BMI categories and docetaxel dose. Conclusions: Obesity, compared with lower weight categories, is associated with improved CSM and OS in mCRPC treated with docetaxel. The mechanistic basis for this observation warrants further study.