Background: The prognostic significance of ABO blood type for lymphoma is largely unknown. We evaluated the prognostic role of ABO blood type in patients with extranodal natural killer (NK)/T-cell lymphoma (ENKTL). Methods: We retrospectively analyzed clinical data of 697 patients with newly diagnosed ENKTL from three cancer centers. The prognostic value of ABO blood type was evaluated using Kaplan-Meier curves and Cox proportional hazard models. The prognostic values of the International Prognostic Index (IPI) and the Korean Prognostic Index (KPI) were also evaluated. Results: Compared with patients with blood type O, those with blood type non-O tended to display elevated baseline serum C-reactive protein levels (P = 0.038), lower rate of complete remission (P = 0.005), shorter progression-free survival (PFS, P < 0.001), and shorter overall survival (OS, P = 0.001). Patients with blood type O/AB had longer PFS (P < 0.001) and OS (P = 0.001) compared with those with blood type A/B. Multivariate analysis demonstrated that age > 60 years (P < 0.001), mass = 5 cm (P = 0.001), stage III/IV (P < 0.001), elevated serum lactate dehydrogenase (LDH) levels (P = 0.001), and blood type non-O were independent adverse predictors of OS (P = 0.001). ABO blood type was found to be superior to both the IPI in discriminating patients with different outcomes in the IPI low-risk group and the KPI in distinguishing between the intermediate-to-low-and high-to-intermediate-risk groups. Conclusions: ABO blood type was an independent predictor of clinical outcome for patients with ENKTL.