Mucocele of the vermiform appendix is caused by mucus retention in its lumen, due to obstruction or hyperproduction. Appendiceal malignancy can be the underlying cause, making accurate preoperative diagnosis imperative. In women, it can sometimes present as an adnexal mass. A rare case of an appendiceal mucocele is presented, mimicking a cystic tumor of the right adnexum, both clinically and ultrasonographically. In addition, serum levels of CA-125 were increased. This is the first case of a mucocele of the appendix simulating an adnexal mass on ultrasound with increased levels of CA-125 to be reported. This clinical entity should be considered in patients presenting on ultrasound with a right-sided adnexal mass as a rare potential diagnosis.