A 71-year-old female complained of abdominal pain, weight loss and abdominal distension. Gynecologic examination revealed a hardly movable, palpable mass in the lower abdomen, reaching the umbilicus. An abdominal ultrasound and computed tomography (CT) scan suggested a large abdominal mass with the possible origin in the left ovary and without significant lymph node enlargements. The patient subsequently underwent complete evacuation of tumor tissue, omentectomy and total abdominal hysterectomy and bilateral salpingo-ovariectomy. Immunohistochemical examination revealed strongly positive staining of tumor cells for CD117. The final pathologic diagnosis was a primary extragastrointestinal stromal tumor (EGIST) of the sigmoid mesocolon with omental metastasis. The differential diagnosis of the tumor presented in the lower abdomen should consider the EGIST as well.