Objective: We aimed to estimate 5-year conditional relative survival (5Y CRS) rates of endometrial cancer in Korea accounting for time already survived. We also presented subgroup-specific estimates stratified by various patient characteristics. Methods: Using the data from the Korean Central Cancer Registry, we calculated 5-year CRS rates in patients who were diagnosed with endometrial cancer between 1998 and 2017. The CRS rates were presented by year of diagnosis, age at diagnosis, histology, cancer stage, and treatment received. Results: The 5-year relative survival rate at the time of diagnosis was 89.0% for all cases. The probability of surviving an additional 5 years (i.e., 5Y CRS), if the patient survived 1, 2, 3, 4, and 5 years after diagnosis was 91.8, 94.1, 95.6, 96.5, and 97.3%, respectively. Patients with poor initial prognoses, i.e., those who were older, had non-endometrioid histology, and high stage, showed the largest improvements in 5Y CRS, reaching >90% for most subgroups, except those with serous histology (88.4%) and distant stage (77.7%). Patients aged ≥70 years had the highest probability of death in the 1^st and 2^nd years after diagnosis (13.8 and 11.0%), but the conditional probability of death in the 3^rd, 4^th, and 5^th years declined rapidly to 7.3%, 4.5%, and 3.7%, respectively. Conclusions: The CRS rates for patients with endometrial cancer improved with increased time elapsed from diagnosis. The greatest improvements in 5Y CRS were observed among patients who were older, those with non-endometrioid histology, and those with more advanced disease.