Misdiagnosis of endocervical adenocarcinoma (EAC) as endometrial endometrioid carcinoma (EEC) is one of the major concerns when evaluating endometrial curettage specimens. It is difficult to differentiate EAC involving the endometrium from EEC, particularly when the specimens have only a few small tumor fragments. We report a case of endocervical adenocarcinoma <i>in situ</i> (AIS) with multifocal microscopic involvement of the endometrium. The endometrial curettage specimen obtained from an 82-year-old woman consisted of a large volume of blood and fibrin, with small endometrial tissue fragments showing microscopic foci of atypical glandular proliferation. Based on the presence of complex glands with stratified mucin-poor columnar epithelium and intermediate-grade nuclear atypia, a preoperative diagnosis of grade 1 EEC was made. However, the hysterectomy specimen revealed an endocervical AIS involving the endocervix and low uterine segment. Frequent mitotic figures and apoptotic bodies, characteristic of AIS, were present. The endometrium showed a few microscopic foci of atypical glandular proliferation involving the surface only. Their histological features were similar to those of the endocervical AIS. Immunohistochemically, the atypical glands exhibited block p16 positivity. The final diagnosis was a superficially spreading endocervical AIS with multifocal microscopic involvement of the endometrial surface epithelium. In summary, small tumor tissues in an endometrial curettage may lead to misdiagnosis of AIS or EAC as EEC, especially when the pathologists are unaware of the possibility of microscopic endometrial involvement of AIS or EAC. The origin of the tumor can be correctly determined based on a combination of histological features and immunostaining. Endocervical AIS involving the endometrium should be included in the differential diagnosis of neoplastic glandular lesions in endometrial curettage specimens. An accurate diagnosis in these cases is important because of its significant implications for clinical management.