Tumor-infiltrating neutrophils (TIN) carry out quite significant but opposite functions in different cancers, and their function in biliary cancer has not been fully characterized. To investigate the prognostic significance of TIN in biliary cancer, a training set (n = 118) and a validation set (n = 127) were involved in this study. TIN were evaluated by immunohistochemical staining of CD66b, and then defined as low (neutrophils <18/high-power field [HPF]) vs high (neutrophils ≥18/HPF). Kaplan-Meier curve, Cox proportional hazards models and receiver operating characteristic curve were used to assess the prognostic significance. TIN was identified as an independent prognostic factor for overall survival in the training set (HR: 4.720; 95% CI: 2.623-8.493; P < .001) which was confirmed in the validation set (HR: 4.993; 95% CI: 2.626-9.492; P < .001). Notably, among patients with stage III and IV disease, those with low TIN could benefit from adjuvant chemotherapy, with a reduced risk of compromised survival compared with those with high TIN (HR: 0.294; 95% CI: 0.099-0.873; P = .047 in the training set; and HR: 0.100; 95% CI: 0.022-0.462; P = .006 in the validation set). In addition, TIN were negatively related to biological pathways as regulation of activated T-cell proliferation and lymphocyte-mediated immunity, and showed a negative correlation with CD8 +  T cells (r = -.324, P < .001). Taken together, our results implicate TIN as an independent marker of prognosis and indicator of patients who would benefit from adjuvant chemotherapy in biliary cancer.