Endometrial carcinosarcoma (CS), or malignant mixed Mullerian tumor, is a high-grade variant of endometrial carcinoma with a high risk for recurrence. Adjuvant multi-agent chemotherapy (CHT) is a preferred treatment modality in all stages of disease, with or without radiotherapy (RT). As prospective data are lacking regarding the benefit of RT when added to adjuvant CHT in CS, we sought to analyze the Surveillance, Epidemiology, and End Results (SEER) database to ascertain whether RT improves overall survival (OS) and cancer-specific survival (CSS) when added to surgery and CHT for endometrial CS. SEER 18 Custom Data registries (Nov 2018 submission) were queried for endometrial (ICD10 C54.1-9, C55.9) CS (ICD-0-3 8980-3). Patients with stage I-III CS who received CHT with or without RT were included for analysis. Univariate analysis (UVA) and multivariable analysis (MVA) using Kaplan-Meier and Cox proportional hazards regression modeling were performed. Propensity-score matched analysis with inverse probability of treatment weighting was performed to account for indication bias. Furthermore, conditional landmark analysis using a minimum follow-up time of 3 months was performed to minimize immortal time bias. A total of 1,541 patients treated from 1988-2016 were included in this analysis. Median age at diagnosis was 65 (IQR 59-71). Patients had FIGO stage IA (30%), IB (14%), II (9%), IIIA (8%), IIIB (3.6%), or IIIC (32%) disease. All patients received CHT and 54% received RT, either with external beam RT (EBRT) alone (25%), EBRT with a brachytherapy boost (9.7%), or brachytherapy alone (18.7%). On UVA, adjuvant RT improved OS (HR 0.65, 95% CI 0.56-0.77, p<0.001) and CSS (HR 0.64, 95% CI 0.54-0.76, p<0.001) compared to surgery and CHT alone. Of note, RT was more likely to be delivered to patients who were slightly younger (p = 0.034) and to those with a lower FIGO stage (p<0.001). After adjusting for these baseline imbalances, the OS benefit provided by adjuvant RT persisted (HR 0.76, 95% CI 0.65-0.89, p<0.001), as did the CSS benefit (HR 0.74, 95% CI 0.63-0.87, p<0.001). Moreover, these findings were maintained using a 3-month conditional landmark analysis (HR 0.67, 95% CI 0.57-0.79). On subgroup analysis, factors associated with an OS benefit included stage IA (HR 0.45, 95% CI 0.45-0.99) and IIIA (HR 0.42, 95% CI 0.24-0.74) disease, grade III disease (HR 0.78, 95% CI 0.61-0.99), ages 50-64 years (HR 0.76, 95% CI 0.59-0.98) and ≥65 years (HR 0.79, 95% CI 0.64-0.98), and treatment delivered between 2010-2016 (HR 0.65, 95% CI 0.52-0.81). In patients with FIGO stage I-III endometrial CS undergoing surgery and CHT, RT improves OS and CSS. Further prospective data are needed to investigate the benefit of RT in endometrial CS treated with modern therapies.