Appendiceal mucoceles are usually associated with hyperplastic or neoplastic mucosal proliferation and obstructive lesions such as postinflammatory scarring, fecalith, carcinoid tumor, and endometriosis. Among these, an association with endometriosis is known to be very exceptional. We herein report on a rare case of obstructive mucocele of the appendix that was secondary to endometriosis in a 42-year-old patient with pelvic endometriosis. A computed tomography scan demonstrated a periappendiceal abscess-like lesion with a left adnexal mass that was suggestive of endometriosis. On gross examination, the periappendiceal lesion consisted of a mucin-filled cavity (the so-called mucocele) that was 1.8 cm in diameter, and it protruded into the cecal lumen. Microscopically, the lining epithelium of the cavity was almost totally denuded. A small amount of mucus spilled over outside the mucocele, but pseudomyxoma peritonei was not present. The wall of the mucocele showed the characteristic multiple foci of endometriosis involving predominantly the muscularis propria and the serosa of the appendix and adjacent cecal walls.