Abstract Background/purpose To perform a large analysis of Stage I endometrioid-type endometrial cancer patients to determine the impact of adjuvant radiotherapy (ART) on survival. Material/methods 132,976 FIGO Stage I endometrioid-type endometrial cancer patients treated surgically were identified within the National Cancer Database (NCDB) comprising Commission on Cancer facilities in the United States. Patients were categorized as observation (OBS) or ART (vaginal brachytherapy, external beam radiotherapy, or both). Univariable generalized linear mixed effects models were used to estimate the odds of receiving ART, and a multivariable frailty survival model was used to estimate the instantaneous hazard of death for those receiving OBS versus ART. Due to the presence of a significant interaction, these estimates were stratified by PORTEC-based low, low-intermediate, high-intermediate, and high risk groups. Results 104,645 (79%) underwent OBS while 28,331 (21%) received ART. Of those receiving ART, 12,913 (46%) received VBT alone, 12,857 (45%) received EBRT alone, and 2561 (9%) received EBRT + VBT. On univariable analysis, increasing stage/myometrial invasion, higher grade, older age, presence of lymphovascular space invasion, and larger tumor size predicted poorer survival (all p < 0.01). On multivariable analysis, patients at high-intermediate risk and high risk experienced improved survival with ART with a hazard ratio of 0.796 (95% CI: 0.731–0.867; p < 0.001) and 0.783 (95% CI: 0.693–0.885; p < 0.001), respectively. There was no survival benefit for ART among patients at low or low-intermediate risk. Conclusions In Stage I high-intermediate and high risk endometrioid-type endometrial cancer patients, ART significantly improves overall survival.