Background: Many patients with muscle-invasive bladder cancer are not ideal candidates for chemotherapy or radical cystectomy. Moreover, local progression may further impair quality of life. Bladder preservation with maximal transurethral resection of bladder tumor (TURBT) and stereotactic body radiotherapy (SBRT) is a viable treatment option for such patients, offering durable local control while potentiating the effect of immunotherapy. Herein, we report on the safety profile of SBRT in combination with durvalumab (anti-PDL1) and tremelimumab (anti-CTLA4) in the safety lead-in cohort of an institutional Phase II trial for cisplatin-ineligible, unresectable locally advanced or metastatic bladder cancer patients. Methods: Patients with locally advanced or metastatic urothelial carcinoma, who were unwilling or unable to undergo radical cystectomy and/or systemic chemotherapy were eligible. Patients received maximal TURBT, followed by durvalumab 1500mg IV every 4 weeks and bladder SBRT 33Gy in 5 fractions between the first 2 cycles. After 6 such patients did not experience a dose limiting toxicity, tremelimumab 75mg IV was added for cycles 1-2. Patients were followed with CT Chest/Abdomen/Pelvis and cystoscopy every 3 months. Treatment-related adverse events (TRAEs) were assessed per CTCAE 5.0 and progression was assessed per RECIST 1.1. Results: Between January 2019 and May 2020, 10 patients received TURBT followed by SBRT and durvalumab (Age 73-96, 3 Female, 7 ECOG 1, 3 ECOG 0). Five patients had localized disease and 5 had nodal or distant metastases. Median cycles of durvalumab was 12.5 (2-13), and 4 patients received 2 cycles of tremelimumab. At a median follow-up of 16.8mo (6.4-30mo), there were 2 Grade 3+ immunotherapy-related TRAEs (1 Grade 3 lipase elevation self-resolved, 2 Grade 3 myositis treated with steroids). The most common TRAEs of any grade included pancreatic enzyme elevation (3 patients), rash (3 patients), and urinary symptoms related to RT (3 patients). Disease control rate 70% and local control was 90%. There were 3 non-treatment related deaths. Conclusions: Bladder SBRT with durvalumab and tremelimumab was well tolerated and demonstrated promising local control in the safety lead-in cohort. Clinical trial information: NCT03601455.