Objective: To test the effect of obesity (body mass index ≥30 kg/m2) on perioperative outcomes and total hospital charges at robot-assisted vs open radical prostatectomy (RARP vs ORP). Methods: Within the National Inpatient Sample database (2008-2015), we identified obese vs nonobese RARP and ORP patients. Estimated annual percent changes, multivariable logistic regression and linear regression models were used. All models were adjusted for clustering and weighted. Results: Of all, 53,626 (60%) underwent RARP vs 35,757 (40%) underwent ORP. At RARP, 8.6% were obese vs 6.9% at ORP. RARP rate increased significantly over time (12.5%-81.5%). Obesity rate increased significantly over time at both, RARP (5.1%-10.5%) and ORP (5.4%-10.7%). In multivariable logistic regression models, obesity predicted 5 of 11 unfavourable perioperative complications at RARP (odds ratio: 1.6-1.8) and 9 of 11 at ORP (odds ratio: 1.3-2.8). In linear regression models, obesity significantly added to total hospital charges at RARP (740$) and ORP (312$). Conclusion: Obesity may predispose to higher rates of adverse outcomes at RP. Its effect varies according to surgical approach. © 2019 Elsevier Inc.