Endometriosis is defined as the presence of ectopic functional endometrial tissue outside the uterine cavity. It most commonly occurs in the pelvic organs, including the ovaries, ligaments of the uterus, and the pouch of Douglas. Extrapelvic implantation of endometrial tissue has also been reported in the literature. Extrapelvic endometriosis can be explained by lymphatic/vascular migration or mechanical transplantation of the tissue during surgery. Rectus abdominis muscle endometriosis is a rare phenomenon that usually presents with a palpable abdominal mass and cyclic pain. Ultrasonography (USG), computed tomography (CT), and magnetic resonance imaging (MRI) can be effective to define the location and size of the mass. Our patient was a 32-year-old woman presenting with cyclic abdominal pain and the development of an abdominal mass she had noticed for a year. USG and MRI scans revealed an endometrial focus in the right rectus abdominis muscle. Surgical excision with negative margins was performed, since surgical treatment of the lesion is offered as the definitive treatment.