Background: Metastatic urothelial cancer is aggressive and associated with dismal 5-yr overall survival. Platinum-based chemotherapy and checkpoint inhibitors are standard first-line options with enfortumab vedotin, sacituzumab govitecan, and erdafitinib (in select FGFR altered tumors) subsequently utilized upon disease progression. However, despite these options, long-term outcomes remain poor, and novel strategies are needed to improve oncologic outcomes. We hypothesized that combining avelumab (anti-PD-L1 immunotherapy) with docetaxel is safe and will enhance cancer cell death by releasing neoantigens and potentiating anti-tumor immune-mediated cytotoxicity. Methods: This is a phase 1b, single-arm, open-label prospective clinical trial evaluating the combination of avelumab with docetaxel in adult subjects with locally advanced or metastatic urothelial carcinoma with disease progression during or following platinum-containing chemotherapy or within 12 months of neoadjuvant or adjuvant platinum-containing chemotherapy. There are two phases. In Phase 1b, dose de-escalation of docetaxel (Level 0: 75, Level -1: 60 or Level -2: 45 mg/m2) with standard dose avelumab (10 mg/kg) aimed to establish the phase 2 dose in a standard 3+3 design. In dose-expansion, avelumab with the RP2D of docetaxel was evaluated for efficacy. The combination therapy is administered every 3 weeks for 6 cycles, and then avelumab alone is continued every 2 weeks. The primary endpoint is safety. Efficacy endpoints include objective response rate (ORR), progression-free survival (PFS) and overall survival (OS). Results: At the cutoff date of 10/8/2021, 21 patients were enrolled in the study. Only one of the 6 patients treated with level 0 dose of docetaxel had dose-limiting toxicity (neutropenic fever). Docetaxel at 75 mg/m2 along the avelumab was deemed safe for dose expansion cohort. An additional 15 patients were enrolled in the dose-expansion cohort. Of the 20 evaluable patients, ORR (CR+PR) was seen in 70% of subjects. (CR: 30%, PR: 40%, SD: 5%, PD: 25%, and 1 patient was not evaluable). The median PFS was 9.2 months (range: 1.5 – 25.8 months), and median OS was not reached. The most common Grade 3 or 4 AEs were febrile neutropenia, transaminitis, diarrhea, anemia, and neutropenia. No treatment-related deaths were noted. Conclusions: The combination of avelumab with docetaxel is safe with promising efficacy that is worth further studying in patients with platinum-refractory or ineligible metastatic urothelial cancer. Clinical trial information: NCT03575013.