© Copyright 2022, Mary Ann Liebert, Inc., publishers 2022.Objective: The aim of the study was to assess whether severely obese patients have an increased risk of complications during and after retrograde intrarenal surgery (RIRS). Materials and Methods: The data of 639 consecutive patients undergoing RIRS for the treatment of upper tract urinary stones were analyzed retrospectively. The patients were divided into two groups according to their body mass index numbers (Group 1, <35; Group 2, ≥35). The patients' demographics, stone characteristics, operative outcomes, and complication rates were compared between the groups. The primary objective was to examine whether the intraoperative and postoperative complication rates were higher in patients with a body mass index of ≥35 kg/m2. Results: After matching of confounding factors, Group 1 comprised 135 patients, and Group 2 comprised 47 patients. The baseline characteristics were similar between the groups. There were no significant differences between groups for intraoperative complication rates (11.8% and 12.8%, respectively; p = 0.97). There was statistically significant difference in favor of Group 2 for postoperative complication rates (12.6% and 29.7%; respectively, p < 0.01), overall complication rates (22.9% and 38.2%; respectively, p = 0.02), mean operation time (56.15 vs 66.45 minutes; respectively, p = 0.01), and length of stay (1.4 vs 2.1 days; p = 0.03). Stone-free rates (75.5% vs 85.1%; respectively, p = 0.17) did not differ between groups. Conclusions: RIRS is an efficient and feasible treatment option for upper urinary tract stones in severely obese patients. However, higher possibility of postoperative, especially infectious, complication rates should be considered in these patients.