Background: Tucidinostat (also known as chidamide), a novel histone deacetylase (HDAC) inhibitor, has been approved for treatment of patients (pts) with relapsed or refractory peripheral T-cell lymphoma and advanced hormone receptor-positive breast cancer in China. Tislelizumab (a PD-1 checkpoint inhibitor) is currently approved for pts with PD-L1-high locally advanced or metastatic urothelial carcinoma (UC) who progressed during/following platinum-containing therapy in China. However, the response rate of tislelizumab as monotherapy is limited in UC. The aim of this study was to evaluate the efficacy and safety of tucidinostat combined with tislelizumab in pts with locally advanced or metastatic UC. Methods: This study is an open-label, single-center, phase Ⅱ trial. Patients who previously failed systemic platinum-based chemotherapy with UC were included, regardless of PD-L1 status. Eligible pts received 30 mg oral tucidinostat twice weekly (BIW) in combination with 200 mg tislelizumab by intravenous drip every three weeks (Q3W). This is a Simon's two-stage design. All pts were treated until progression or unacceptable toxicity in two years. The primary endpoint was objective response rate (ORR). The secondary endpoints included progression free survival (PFS), disease control rate (DCR), overall survival (OS), the safety and tolerability. Results: From Jan. 2021 to Sep. 2022, 26 pts were enrolled (median age 62 y, range 31-74y), of whom 24 were evaluable for response per RECIST 1.1 criteria. One pt was lost to follow-up and one withdrew the group due to sudden cerebral infarction. Complete response (CR) was observed in 2 pts, while partial response (PR) was observed in 8 pts, with an ORR of 41.7% (10/24). The DCR was 62.5%. The median PFS was 4.6 months. Most adverse events (AEs) were grade 1 or 2. The most common grade 3 AEs were neutropenia 11.5%, thrombocytopenia 11.5%, leukopenia 7.7%, anemia 7.7%, anorexia 7.7%, creatinine increased 3.8%. One pt experienced grade 4 immuno-pneumonia was permanently discontinued. One pt was on maintenance therapy of tucidinostat without immunotherapy because of interstitial pneumonia. Conclusions: The preliminary data indicate that tucidinostat combined with tislelizumab might be a potentially effective regimen with a manageable safety profile in UC. The study remains ongoing and updated results will be presented later. Clinical trial information: NCT04562311.