Alveolar soft part sarcoma (ASPS) is a rare tumor of uncertain histogenesis, mainly localized in the extremities. ASPS originating in the uterine corpus is quite rare; only eight such cases have been reported in the literature. We here present another case of ASPS found in the endometrium in a 50-year-old woman. Metastatic malignant tumor, including ASPS from other organs, was excluded by physical examination and imaging modalities. Thallium 201 was only localized in the uterus. The tumor showed characteristic histological features of ASPS: alveolar architecture with fibrovascular septa and abundant eosinophilic granular cytoplasm with periodic acid-Schiff-positive crystalline material. Diffuse nuclear immunoreactivity for TFE3, a marker recently reported to be specific for ASPS, further supported the diagnosis of ASPS. Interestingly, this tumor was negative for myogenic markers, but positive for CD10, progesterone receptor, and estrogen receptor. These immunohistochemical results and the tumor location suggest a possible link between endometrial stromal cells and the development of this tumor.