Introduction The purposes of this study were to compare adjuvant treatment modalities and to determine prognostic factors in stage III endometrioid endometrial cancer (EC). Methods SATEN III was a retrospective study involving 13 centers from 10 countries. Patients who had been operated on between 1998 and 2018 and diagnosed with stage III endometrioid EC were analyzed. Results A total of 990 women were identified; 317 with stage IIIA, 18 with stage IIIB, and 655 with stage IIIC diseases. The median follow-up was 42 months. The 5-year disease-free survival (DFS) of patients with stage III EC by adjuvant treatment modality was 68.5% for radiotherapy (RT), 54.6% for chemotherapy (CT), and 69.4% for chemoradiation (CRT) (p=0.11). The 5-year overall survival (OS) for those patients was 75.6% for RT, 75% for CT, and 80.7% for CRT (p=0.48). For patients with stage IIIA disease treated by RT versus CT versus CRT, the 5-year OS rates were 75.6%, 75.0%, and 80.7%, respectively (p=0.48). Negative peritoneal cytology (HR: 0.45, 95% CI: 0.23 to 0.86; p=0.02) and performance of lymphadenectomy (HR: 0.33, 95% CI: 0.16 to 0.77, p=0.001) were independent predictors for improved OS for stage IIIA EC. For women with stage IIIC EC treated by RT, CT, and CRT, the 5-year OS rates were 78.9%, 67.0%, and 69.8%, respectively (p=0.08). Independent prognostic factors for better OS for stage IIIC disease were age <60 (HR: 0.50, 95%CI: 0.36 to 0.69, p<0.001), grade 1 or 2 disease (HR: 0.59, 95% CI: 0.37 to 0.94, p=0.014; and HR: 0.65, 95%CI: 0.46 to 0.91, p=0.014, respectively), absence of cervical stromal involvement (HR: 063, 95% CI: 0.46 to 0.86, p=0.004) and performance of para-aortic lymphadenectomy (HR: 0.52, 95% CI: 0.35 to 0.72, p<0.001). Discussion Although not statistically significant, CRT seemed to be a better adjuvant treatment option for stage IIIA endometrioid EC. Systematic lymphadenectomy seemed to improve survival outcomes in stage III endometrioid EC.