Introduction We aimed to evaluate the efficacy and toxicity of the combination of 6 cycles of chemotherapy and radiation therapy compared with chemotherapy alone as postoperative adjuvant therapy for patients with stage III endometrial cancer. Methods This retrospective cohort study included patients with stage III endometrial cancer who received postoperative chemoradiotherapy or chemotherapy alone at 6 hospitals between January 2009 and December 2019. The progression-free survival (PFS) and overall survival (OS) for each treatment group were analyzed using the Kaplan–Meier method. We also assessed differences in toxicity profiles between the treatment groups. Results A total of 133 patients met the inclusion criteria. Of these, 80 patients (60.2%) received adjuvant chemoradiotherapy and 53 (39.8%) received chemotherapy alone. The PFS and OS did not differ significantly between the groups. For patients with stage IIIC endometrioid subtype, the chemoradiotherapy group had significantly longer PFS rate than did the chemotherapy alone group (log-rank test, P = .019), although there was no significant difference in the OS (log-rank test, P = .100). CRT was identified as a favorable prognostic factor for PFS in multivariate analysis (adjusted HR, .37; 95% CI, .16-.87; P = .022). Patients treated with chemoradiotherapy more frequently suffered from grade 4 neutropenia (73.8% vs 52.8%; P = .018) and grade 3 or worse thrombocytopenia (36.3% vs 9.4%; P = .001) compared with the chemotherapy alone group. There were no differences between the 2 treatment groups in the frequency of toxicity-related treatment discontinuation or dose reduction. Conclusion We confirmed that chemoradiotherapy yields longer progression-free survival than does chemotherapy alone for patients with stage IIIC endometrioid endometrial cancer, with an acceptable toxicity profile.