Objective@#To investigate the incidence and survival rates of primary uterine carcinosarcoma (UCS) in Korea. @*Methods@#From the Korea Central Cancer Registry, we identified patients diagnosed with primary UCS between 1999 and 2018 and collected their information, including age at diagnosis, Surveillance, Epidemiology, and End Results (SEER) summary stage, and treatment. Age-standardized incidence rates (ASRs) and annual percent changes (APCs) were calculated. Baseline characteristics and overall survival (OS) were compared by study periods, ages, and stages at initial diagnosis. @*Results@#Overall, the incidence rate of primary UCS increased markedly during the time period: ASRs, 0.02 per 100,000 in 1999 and 0.25 per 100,000 in 2018 (APC, 13.9%; p<0.001). No difference in OS was observed between patients diagnosed in 1999–2008 and those diagnosed in 2009–2018 (5-year survival rate, 46.0% vs. 48.6%; p=0.871). Considering the mean patient age at diagnosis of UCS, we divided the study population into 2 groups. Patients aged ≥60 years had a more frequent prior radiation history, received less multi-modality treatment, and showed worse OS than those aged <60 years (5-year survival rate, 42.7% vs. 53.6%; p=0.001). In multivariate analysis, both old age at diagnosis (≥60 years) and the SEER summary stage were identified as independent poor prognostic factors for OS, whereas radiation history before the diagnosis of UCS was not. @*Conclusion@#The incidence rate of UCS in Korea increased significantly from 1999 to 2018. Advanced stage and old age (≥60 years) at diagnosis might be poor prognostic factors for survival, but not prior radiation history.