Background: Advanced biliary tract cancers (BTCs) are rare and aggressive malignancies with limited treatment options. Recently, immune checkpoint inhibitors in combination with chemotherapy (ICI-C) gained approval as first-line therapy for unresectable or metastatic BTCs. Two large phase III clinical trials of ICI-C in BTCs did not show a significant correlation between PD-L1 expression and therapeutic response. However, in a phase II study of single-agent nivolumab, there was an association. In light of these conflicting findings, we conducted a comprehensive systematic review and meta-analysis of clinical trials investigating ICI-C in BTCs to assess whether PD-L1 expression could predict treatment response. Methods: We searched PubMed, Web of Science, EMBASE, and Cochrane databases to identify relevant studies. Patient characteristics, method of PD-L1 assessment, and measures of treatment response including hazard ratio (HR), objective response rate (ORR), progression-free-survival (PFS), and overall survival (OS) were extracted. Pooled ORR and HR were calculated using random effect model and assessed for study heterogeneity. Results: Among the 4,882 studies screened, seven clinical trials evaluating the efficacy of ICI-C in BTCs were identified. Of these trials, two randomized phase III trials report HR between PD-L1 positive and negative patients, while five phase II clinical trials reported ORR based on PD-L1 expression. Out of the 1,069 patients included in these trials, 954 underwent assessment for PD-L1 expression. 66.4% (n=634) exhibited PD-L1 expression level ≥ 1% as determined by either tumor proportion score (TPS) or combined positive score (CPS), while 33.5% (n=320) were classified as PD-L1 negative. Notably, 115 patients were not tested for PD-L1 expression, a variety of PD-L1 assays were used across the studies, and only some studies reported multiple PD-L1 cutoffs. Pooled analysis of two-phase III trials demonstrated that PD-L1 expression ≥ 1% was associated with improved OS (pooled HR 0.83, 95% CI 0.72-0.95) compared to PD-L1 < 1% (pooled HR 0.85, 95% CI 0.67-1.07). Additionally, pooled ORR from five phase II studies showed a trend towards an improved response in patients with PD-L1 expression ≥ 1% (pooled ORR 64%, 95% CI 52-74%) compared to PD-L1 <1% (pooled ORR 46%, 95% CI 31-62%) with a subgroup difference p-value of 0.08. Conclusions: This meta-analysis suggests that BTCs patients with PD-L1 expression ≥ 1% may exhibit a more favorable response to chemo-immunotherapy, and that a majority of BTCs have this level of expression. These findings support further investigation of the use of PD-L1 as a biomarker, and in particular investigation of different PD-L1 levels, which may inform treatment for patients with relative contraindications to immunotherapy use.