Mullerian cyst of the uterus, also known as endosalpingiosis, is rare and often can be misdiagnosed as adnexal cyst on pelvic sonography. A 47-year-old woman was referred for an incidental finding of a right adnexal mass on pelvic sonography, suspected to be an ovarian cyst. During laparoscopy, a pedunculated cystic mass arising from the right anterior uterine wall was seen. The mass was resected laparoscopically and histologic examination showed a benign Mullerian cyst of the uterus. Diagnosis of Mullerian cyst of the uterus can be challenging. However, with increased awareness, preoperative diagnosis of this condition should be possible sonographically.