Dedifferentiated endometrial carcinoma is an uncommon but aggressive type of endometrial cancer that may pose a variety of diagnostic challenges, the most clinically significant being its potential for misclassification as lower-grade endometrioid adenocarcinoma. Its behavior and diagnostic features have come to attention in the past decade, leading to its recognition as a distinct entity in the 2014 update of the World Health Organization classification of endometrial cancer. Dedifferentiated endometrial carcinoma is defined as a tumor composed of a mixture of undifferentiated carcinoma and conventional endometrioid adenocarcinoma. Undifferentiated carcinoma grows as sheets of noncohesive, monomorphic, moderately atypical tumor cells that have either completely or nearly completely lost Mullerian epithelial immunophenotype (epithelial membrane antigen, keratin, PAX8, estrogen receptor) and E-cadherin immunoexpression. Extensive geographic necrosis is common, and many cases exhibit foci of tumor cells with rhabdoid morphology. In contrast, the accompanying component of differentiated carcinoma exhibits the typical morphology and immunophenotype of grade 1 or 2 endometrioid adenocarcinoma. Regardless of the percentage of tumor comprised of the undifferentiated carcinoma, patients usually present with advanced-stage disease and progress to recurrence and/or death within a short timeframe. The behavior is more aggressive than conventional grade 2 or grade 3 endometrioid adenocarcinoma, which are the 2 most common misdiagnoses of dedifferentiated endometrial carcinoma. Loss of immunohistochemical expression of mismatch repair proteins is common; however, there are conflicting data on whether this is significantly more common than in other subtypes of endometrial carcinoma. Although the pathogenesis of dedifferentiated endometrial carcinoma remains to be fully elucidated, recent studies suggest a role for inactivation of SMARCA4 or SMARCB1, core units of the SWI/SNF chromatin remodeling complex that are also inactivated in other aggressive poorly differentiated tumors with rhabdoid morphology such as ovarian small cell carcinoma, hypercalcemic type, among others. Loss of SMARCA4 immunoexpression in the undifferentiated carcinoma component occurs in up to half of dedifferentiated endometrial carcinomas; since it is retained by the differentiated component, as well as by conventional grade 3 endometrioid adenocarcinomas, SMARCA4 may emerge as a useful tool in the differential diagnosis, although its specificity remains to be fully determined. This review discusses the clinical, morphologic, and immunohistochemical features of dedifferentiated endometrial carcinoma, as well as a practical approach to its differential diagnosis.