Background: Appendicitis in a pelvic appendix can be difficult to diagnose. Transvaginal sonography may help to visualize an inflamed pelvic appendix.Case: A 31-year-old woman presented to the hospital with symptoms suggestive of pelvic inflammatory disease. Transabdominal ultrasound, useful in the diagnosis of appendicitis, showed a mass between the uterus and the right ovary. Transvaginal ultrasound clarified the finding as a bulbous fluid-filled structure extending into the cul-de-sac. The structure was diagnosed as an inflamed appendix. Laparoscopic appendectomy was performed, and the patient had an uneventful recovery.Conclusion: To our knowledge, this is the first case of appendicitis diagnosed with transvaginal sonography. Transvaginal sonography can delineate the features of an inflamed pelvic appendix and help to narrow the diagnostic possibilities in symptomatic women of childbearing age.