Endometriosis is prevalent among women of reproductive age, and is most commonly found in the gynecologic organs themselves and the surrounding pelvic peritoneum. Endometriosis of the appendix, however, is rare. Preoperative diagnosis is difficult and a definitive diagnosis is usually established following histopathological examination of the appendix. We report a case of endometriosis of the appendix in a 29-year-old woman who presented with right lower quadrant abdominal pain. Rebound tenderness was localized to McBurney's point. Her WBC count was 12,300/mm3 and her CRP was 6.497 mg/dl. Ultrasound and computed tomography detected a calcified region inside the cecum and slight thickening of the wall of the appendix. Based on these findings, the patient was diagnosed with acute appendicitis and underwent an appendectomy. The appendix appeared mildly congested, but the mucosa of the appendix was nearly normal and without macroscopic inflammation. Histopathological examination demonstrated ectopic endometrial glands and stroma in the muscularis. These stroma cells were positive for CD10 on immunohistochemical staining, establishing a diagnosis of endometriosis of the appendix. The patient had a good clinical course and no residual pain postoperatively.