Endometrial carcinoma is the most common malignancy of the upper female genital tract but is rare in teenagers. Here, we report the case of a 15-year-old, nulliparous, morbidly obese female with complaints of asthenia and menometrorrhagia lasting for six months. On examination, the patient had an enlarged uterus approximately 14 gestational weeks in size, and ultrasound revealed an intrauterine mass and polycystic ovaries. An endometrial biopsy performed during hysteroscopy revealed endometrioid adenocarcinoma, and magnetic resonance imaging showed myometrial invasion. The patient underwent a laparotomy involving total abdominal hysterectomy, right salpingo-oophorectomy, wedge-shape dissection of the left ovary, and pelvic and para-aortic lymphadenectomy. We analyze the pathogenesis of endometrial carcinoma in this case and discuss the risk factors for endometrial carcinoma, especially in young women. Gynecologists should be vigilant for persistent abnormal uterine bleeding and other signs of endometrial carcinoma in young women, especially those who have risk factors for the disease.