IMPORTANCE The long-term survival benefit for radiotherapy (RT) in early-stage extranodal natural killer/T-cell lymphoma (NKTCL) is not known, and it is unclear whether improved locoregional control (LRC) translates into a survival benefit. OBJECTIVE To investigate the dose-dependent effect and potential survival benefits of RT on the basis of LRC improvements. DESIGN, SETTING, AND PARTICIPANTS Review of clinical data of patients with early-stage NKTCL at 10 institutions in China between 2000 and 2014. Radiotherapy dose as a continuous variable was entered into the Cox regression model by using penalized spline regression to allow for a nonlinear relationship between RT dose and events. Regression analysis was used to assess whether a linear correlation exists between LRC and progression-free survival (PFS) or overall survival (OS). Patients received chemotherapy (CT) alone, RT alone, or a combination. Chemotherapy alone was defined as 0 Gy. MAIN OUTCOMES AND MEASURES The association between LRC and OS or PFS. RESULTS A total of 1332 patients (923 [69%] male; median age, 43 years [range, 2-87 years]) were reviewed. For patients treated with RT, median dose was 50 Gy (range, 10-70 Gy); 996 (86%) received at least 50 Gy, and 164 (14%) received 10 to 49 Gy. The risk of locoregional recurrence, disease progression, and mortality decreased sharply until 50 to 52 Gy. For patients receiving RT, high-dose RT (>= 50 Gy) was associated with significantly better 5-year LRC (85% vs 73%; P <. 001), PFS (61% vs 50%; P =. 004), and OS (70% vs 58%; P =. 04) than low-dose RT (< 50 Gy). Improved LRC with high-dose RT was independent of the RT/CT sequence or initial response to CT. Radiotherapy yielded a dose-dependent effect on LRC (range, 41%-87%), PFS (18%-63%), and OS (33%-71%). Dose-response regression analysis revealed a linear correlation between 5-year LRC and 5-year PFS (correlation coefficient, r = 0.994, P <. 001; determination coefficient, R-2 = 0.988) or 5-year OS (r = 0.985, P =. 002; R-2 = 0.97), which was externally validated using published data. CONCLUSIONS AND RELEVANCE The optimal dose was 50 Gy for patients with early-stage disease. The improved LRC was associated with prolonged survival. These findings emphasize the importance of RT in optimizing first-line therapy, and provide evidence for making treatment decisions and designing clinical trials.