A 49-year-old woman experienced diffuse abdominal pain for 2 hours. Increased bowel sounds and diffuse abdominal tenderness were noted. The laboratory examination was normal. Ultrasonography showed a mass connected to the uterus and anechoic ascites. Non-contrast-enhanced computed tomography (CT) showed a 7-cm, round, hypodense mass in the pelvic cavity abutting the anterosuperior aspect of the uterus, with a hyperdense component inside the mass, and contiguous hyperdense intraperitoneal fluid. Contrast-enhanced CT revealed contrast enhancement of the hypodense mass. A ruptured uterine leiomyoma of the subserosal type causing hemoperitoneum was diagnosed with CT. Emergency exploratory laparotomy confirmed a subserosal uterine leiomyoma with rupture, and 400 cc intraperitoneal blood. Myomectomy was performed. Pathological examination revealed a 4 × 5 × 6 cm^3, well-encapsulated cellular leiomyoma, and a bleeding site with hemorrhage. The patient had an uneventful postoperative course and was discharged on the 5th postoperative day.