Objective: To test whether body mass index (BMI) amongst patients with metastatic castration-resistant prostate cancer (mCRPC) is associated with overall survival (OS) and cancer-specific survival. Methods: Individual patient data from 1577 men with mCRPC treated with docetaxel and prednisone from the control arms of ASCENT2, VENICE, and MAINSAIL were considered. The role of BMI on survival outcomes was investigated both as a continuous and categorical variable (≤24.9 vs. 25–29.9 vs. ≥30 km/m2). BMI ≥ 30 kg/m2 was considered obese. Analyses were adjusted for age, PSA, ECOG performance status, number of metastases and prior treatment. The Cox semi-proportional hazard model was used to predict OS, whereas competing risks regression was used for predicting cancer-specific mortality (CSM). To exclude any possible effect attributable to higher doses of chemotherapy (titrated according to body-surface area), we checked for eventual interactions between BMI and chemotherapy dose (both as continuous-continuous and categorical-continuous interactions). 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. Median follow-up for survivors was 12 months. Of the 1577 patients included, 655 were deceased by the end of the studies. Regarding OS, BMI emerged as a protective factor 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, relatively to normal weight). The protective effect of high 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 the docetaxel dose at any level in our analyses (all p » 0.05). Conclusions: Obese patients with mCRPC had better cancer-specific and overall survival as compared to overweight and normal weight patients. The protective effect of BMI was not related to receiving higher chemotherapy doses. Further studies aimed at elucidating the biological mechanism behind this effect are warranted.