Our previous study demonstrated that Th17 cells increased significantly in patients with hepatitis B virus-related acute-on-chronic liver failure (HBV-ACLF). However, their prognostic role in HBV-ACLF patients remains unknown.Sixty-eight consecutive HBV-ACLF patients were enrolled in this cohort study. Th17 cells were examined using flow cytometry. Disease severity scores were assessed. ROC curves were used to evaluate the value in predicting prognosis. Survival was analyzed using Kaplan-Meier curves. Predictors of mortality were determined by regression analysis.Th17 cells were significantly higher in HBV-ACLF patients compared to patients with chronic hepatitis B and normal controls (both P < .001). Also, Th17 cells were higher in nonsurviving HBV-ACLF patients than in surviving patients (P = .014). Th17 cells were positively correlated with CLIF-Consortium ACLF (CLIF-C ACLF) score (r = 0.240, P = .048). ROC curves showed that the frequency of Th17 cells had accuracy in predicting 90-day prognosis equivalent to MELD, MELD-Na and CLIF-C ACLF scores in HBV-ACLF (P = .34, P = .26, and P = .15, respectively). More importantly, the area under the ROC curve (AUROC) increased when Th17 cells were combined with MELD, MELD-Na or CLIF-C ACLF score than using Th17 cells alone (P = .021, P = .006, and P = .023, respectively). Kaplan-Meier analysis revealed that higher Th17 cells (≥5.9%) were closely associated with poor overall survival in HBV-ACLF (P = .0086). Additionally, multivariate regression analysis showed that the frequency of Th17 cells over 5.9% was an independent predictor of mortality (OR = 0.154, P = .025).Circulating Th17 cells positively correlated with disease severity in HBV-ACLF. The frequency of Th17 cells over 5.9% could serve as a prognostic biomarker for HBV-ACLF patients.