Background: Biliary tract cancer (BTC) is associated with poor prognosis because of its aggressive and heterogeneous nature. Programmed death ligand 1 (PD-L1) has been considered as a novel biomarker for prognosis and response of immune checkpoint inhibitors in various tumors. However, there are limited data reporting on the role of PD-L1 in advanced BTC patients. Methods: We analyzed 186 patients with advanced BTC who received palliative gemcitabine and platinum between May 2010 and December 2019. All patients were evaluated for PD-L1 expression by combined positive score (CPS) positivity. Results: In all 186 patients, the median age was 62 years (range 38-82), and the primary tumor location was intrahepatic cholangiocarcinoma (IH-CCC) in 72 patients (38.7%), extrahepatic (EH)-CCC in 90 (48.4%), and gallbladder (GB) cancer in 24 (12.9%). There were 158 (84.9%) patients with recurrent disease and 28 (15.1%) with metastatic disease. Among the 186 patients, 53 (28.5%) had PD-L1 CPS positivity, and 133 were CPS negative. The median overall survival (OS) of patients with PD-L1 CPS positivity or negativity was 12.1 and 15.4 months, respectively. The median progression-free survival (PFS) in patients with PD-L1 positivity or negativity was 5.7 and 7.1 months, respectively. The OS and PFS were not statistically different between groups. In sub-group analysis, EH-CCC patients with PD-L1 negativity had more favorable OS (17.2 vs. 11.6 months, p= 0.002) and PFS (7.8 vs. 5.4 months, p= 0.005) than those that were PD-L1 negative. However, this finding was not reproduced in patients with IH-CCC or GB cancer. Univariate analysis of the association between PD-L1 expression and OS in patients with advanced BTC indicated that PD-L1 CPS positivity has a prognostic role in sub-populations older than 60 years (HR 1.743, CI 1.001-3.034, p = 0.050), those with EH-CCC (HR 2.449, CI 1.355-4.426, p = 0.003), and those with liver metastasis (HR 2.511, CI 1.362-4.626, p = 0.003). Conclusions: This study demonstrated that PD-L1 expression might be a novel prognostic biomarker in patients with EH-CCC but not for patients with IH-CCC or GB cancer.