Background: Approximately 15% of patients undergoing radical cystectomy (RC) develop benign ureteroenteric strictures. Of these strictures, the majority are located in the left ureter. To lower the rate of strictures, a retrosigmoid ileal conduit has been suggested. Objective: To investigate the feasibility and safety of a retrosigmoid ileal conduit during robot-assisted RC in bladder cancer patients. Design, setting, and participants: This randomized controlled trial included 303 patients from all five cystectomy centers in Denmark from May 2020 to August 2022. Participants were diagnosed with bladder cancer and scheduled for robot-assisted RC with an ileal conduit. Intervention: Intervention group: a retrosigmoid ileal conduit was constructed using approximately 25 cm of the terminal ileum and tunneled behind the sigmoid where the left ureter was anastomosed from end to side. Control group: the conventional ileal conduit ad modum Bricker with individual end-to-side anastomoses. Outcome measurements and statistical analysis: Patients were analyzed by the intention-to-treat approach. Complications within 90 d were categorized using the Clavien-Dindo grading system and compared using Fisher's exact test. Wilcoxon's test was used for pre- and postoperative renal function. Results and limitations: Of the 149 patients randomized for the retrosigmoid ileal conduit (MOSAIC), a total of 137 (92%) patients received the allocated conduit. Postoperative complications were distributed equally between the two groups. The relative risk of Clavien-Dindo complications of grade ≥III was 1.12 (95% confidence interval: 0.96–1.31) in the intervention group compared with the control group. Conclusions: The retrosigmoid ileal conduit with robot-assisted RC was technically feasible. Early postoperative complications were not significantly different when comparing the two groups. Further investigation of long-term complications, including strictures, is needed. Patient summary: We compared a conventional urinary diversion with a longer conduit to prevent constriction from developing in the ureters. The new conduit is feasible and safe within the first 90 d, with no differences in postoperative complications from those of the conventional diversion.