25-hydroxy vitamin D [25-(OH)D] is widely used to determine vitamin D status in clinic. The aim of our study was to evaluate the prognostic value of 25-(OH)D in extranodal NK/T cell lymphoma (ENKTL). Ninety-three (93) ENKTL patients with available serum 25-(OH)D values were enrolled in our study. Vitamin D deficiency is defined as a 25-(OH)D below 50 nmol/L (20 ng/ml). Univariate and multivariate regression analyses were performed to determine independent risk factors for progression-free survival (PFS) and overall survival (OS). Receiver operator characteristic (ROC) curves were plotted, and corresponding areas under the curves (AUC) were calculated to estimate the accuracy of PINK-E (prognostic index of natural killer lymphoma added with Epstein-Barr virus-DNA status) and 25-(OH)D deficiency in ENKTL risk-stratification. Our results suggested that the vitamin D deficiency was an independent inferior prognostic factor for both PFS [hazard ratio (HR), 2.869; 95% confidence interval (CI), 1.540 to 5.346; P = 0.003] and OS (HR, 3.204; 95% CI, 1.559 to 6.583; P = 0.006) in patients with ENKTL. Additionally, we demonstrated that adding 25-(OH)D deficiency to PINK-E score system indeed has a superior prognostic significance than PINK-E alone for PFS [AUC: 0.796 (95% CI: 0.699 to 0.872) vs. 0.759 (95% CI: 0.659 to 0.841), P = 0.020] and OS [AUC: 0.755 (95% CI: 0.655 to 0.838) vs. 0.721 (95% CI: 0.618 to 0.809), P = 0.040]. In conclusion, our study proved that 25-(OH)D deficiency was associated with inferior survival outcome of ENKTL patients.