Background: Biliary tract cancer (BTC) is rare cancer but has a poor prognosis with only 10% of 5-year overall survival (OS). The treatment of advanced BTC remains challenging with limited standard options. With a 15% of incidence, KRAS is one of the most common mutations in BTC. The immune checkpoint inhibitor as salvage therapy has modest activity in BTC. Methods: We conducted molecular profiling including tumor mutational burden in BTC patients with immune checkpoint inhibitors (ICI) as salvage therapy. Programmed death ligand 1 (PD-L1) expression on tumor cells and on tumor-infiltrating lymphocytes (TIL) was also assessed. Results: Between 2020 to 2022, a total of 62 patients were analyzed. The median age was 68 (58.0-73.0). 47 patients (75.8%) received pembrolizumab and 15 (24.2%) received nivolumab as salvage therapy. All patients received gemcitabine plus cisplatin as the frontline therapy, and 53.2% had fluoropyrimidine plus oxaliplatin (FOLFOX) before ICIs. The median lines of prior chemotherapy were 2.5 (2.0-3.0). KRAS mutation was found in 14 patients (22.6%). 28 patients (45.2%) were positive for PD-L1. There was no statistical correlation between KRAS mutation and PD-L1 expression. The median OS and PFS of patients were 5.2 and 3.5 months respectively. There were no statistically significant differences in PFS according to the status of KRAS mutation (Mutant type vs. Wild type) and PD-L1 expression (Positive vs Negative). In subgroup analysis, patients with KRAS mutation and PD-L1 positivity had longer PFS as compared to patients with KRAS mutation and PD-L1 negative (6.5 vs 2.6 months, p=0.047). This finding was not shown in patients without KRAS mutation. Conclusions: The PD-L1 expression might be a novel biomarker for ICIs in BTC patients with KRAS mutation but not in those with the wild type of KRAS.