Background: Immune checkpoint inhibitors (ICIs) targeting programmed cell death-1 (PD-1) or programmed cell death ligand-1 (PD-L1) has shown promising efficacy in multiple cancers including biliary tract cancers (BTCs). However, the data focusing on the efficacy of ICIs in patients with gallbladder cancer (GBC) is still limited. This study aims to assess the efficacy of ICIs in GBC and explore the clinicopathological and molecular markers of ICI benefit. Methods: This was a single center, retrospective study, including 80 GBC patients who had received ICI (anti-PD-1 monoclonal antibody) therapy at Eastern Hepatobiliary Surgery Hospital (Shanghai, China) between January 2016 and December 2020. Tumor samples from 31 of these patients who had previously underwent surgical resection before ICI therapy were obtained and conducted for whole exome sequencing and immunohistochemical analysis. The primary outcome was progression-free survival (PFS). The secondary outcomes were overall survival (OS), objective response rate (ORR) and disease control rate (DCR). The associations between efficacy with clinicopathological factors, genetic variations and PD-L1/CD8 expression were further explored. Results: Of 80 patients with GBC, 36 were male, the median age was 61 years (range, 30-79). The median PFS and OS was 4.5 months (95% CI, 2.99-5.95) and 8.7 months (95% CI, 7.07-10.26) respectively. Multivariate analysis indicated that alcohol intake history (HR, 2.66; 95% CI, 1.31-5.40; p=0.007), extrahepatic metastasis (HR, 2.03; 95% CI, 1.15-3.59; p=0.015), carcinoma embryonic antigen (CEA) level ≥100U/mL (HR, 3.58; 95% CI, 1.65-7.74; p=0.001) and immune-related adverse events (irAEs) (HR, 0.24; 95% CI, 0.10-0.57; p=0.001) were independent prognostic factors for PFS. Extrahepatic metastasis (HR, 2.42; 95% CI, 1.37-4.31; p=0.003), CEA level ≥100U/mL (HR, 2.82; 95% CI, 1.33-5.95; p=0.007) and irAEs (HR, 0.32; 95% CI, 0.14-0.72; p=0.006) were independent prognostic factors for OS. ORR and DCR were 13.75% and 37.5%, respectively. Patients with irAEs (85.7%, OR, 16.00; 95% CI, 3.26-78.61; p<0.001), high CD8⁺ T-cells-infiltrated GBCs (87.5%, OR, 19.83; 95% CI, 2.00-196.38; p=0.011), immune inflamed phenotype (66.7%, OR, 5.60; 95% CI, 1.16-27.08; p=0.032) had higher DCR, while patients with high level of total bilirubin (TBIL>34.2μmol/L) (8.3%, OR, 0.12; 95% CI, 0.01-0.97; p=0.048) had lower DCR. Patients with high CD8⁺ T-cells-infiltrated or immune inflamed GBCs also had a notably improved PFS and OS. Conclusions: ICIs were effective in patients with GBC. Alcohol intake history, extrahepatic metastasis, CEA level ≥100U/mL, irAEs, high positivity of CD8⁺ T-cells and immune inflamed phenotype may be useful for predicting the efficacy of ICIs in GBC.