Endometrial mesonephric-like adenocarcinoma (MLA) is a rare and aggressive subtype of endometrial carcinoma defined by the 2020 WHO Classification. The corpus uteri dedifferentiated variant is extremely rare with only two cases reported worldwide. We report a 70-year-old female patient initially misdiagnosed with high-grade endometrial stromal sarcoma on curettage, Reassessment showed only undifferentiated carcinoma, and hysterectomy revealed classical MLA and undifferentiated carcinoma components with distinct immunophenotypes. Both components harbored identical KRAS p.G12V mutation, while SMARCB1 loss was restricted to the undifferentiated component. This study identifies cooperative KRAS/SMARCB1 alterations driving MLA dedifferentiation, and highlights diagnostic pitfalls of limited biopsy sampling undifferentiated components.