Dedifferentiated endometrioid adenocarcinoma is a recently defined uterine tumor composed of low-grade endometrioid adenocarcinoma and undifferentiated carcinoma. Herein, we present clinicopathologic, morphologic, and immunohistochemical features of 5 cases of dedifferentiated endometrioid adenocarcinoma. All cases which were diagnosed as mixed endometrial adenocarcinoma (endometrioid+undifferentiated carcinoma) or dedifferentiated endometrioid adenocarcinoma between January 2008 and December 2014 were retrieved from the archives of our institution’s pathology department. The median age of the patients was 58 years. Polypoid growth pattern was seen in 3 patients and 2 were diagnosed at advanced stage. All patients received either external radiotherapy, brachytherapy, chemotherapy or an appropriate combination according to the stage. Only one patient died of the disease. Microscopically, there was a sharp demarcation between the two tumor components. The undifferentiated carcinoma component was composed of diffuse sheets of monomorphic cells lacking any differentiation. Focal pleomorphism and rhabdoid features were also noted. The undifferentiated carcinoma component was variably positive for PAX-8, cytokeratin, EMA, estrogen receptor, and neuroendocrine markers. Misdiagnosis of undifferentiated carcinoma in dedifferentiated endometrioid adenocarcinoma as grade 3 endometrioid adenocarcinoma is not uncommon. The recognition of morphologic and immunohistochemical features of this newly described entity is crucial because it alters treatment and prognosis.