Isolated torsion of fallopian tube is a rare cause of acute pelvic pain. This entity is difficult to diagnose preoperatively due to nonspecific clinical presentation and a definitive diagnosis is always made on surgical exploration. A 65-year-old woman was admitted to Katihar Medical College and Hospital with chief complaints of acute pelvic pain, nausea and vomiting. On abdominal examination, there was marked tenderness of right iliac fossa. Vaginal examination revealed a bulky uterus whose mobility was restricted by a very tender, oval, firm, mobile mass in the right fornix. Ultrasound revealed a well-circumscribed right adnexal cystic mass of 84 x 70 x 46 mm with thick septations and lobulations. Surgical exploration revealed a gangrenous right hydrosalpinx. Total abdominal hysterectomy with bilateral salpingo-oophorectomy was done.