Endometriosis is a benign condition in which endometrial glands and stroma are present outside the uterine cavity. The most common sites are the ovaries. Abdominal wall implantation, although uncommon, typically occurs in areas of previous surgical scars, usually involving the subcutaneous tissue. Endometriosis arising in the rectus abdominis muscle is rare. A case of extra pelvic endometriosis, spontaneously arising within the rectus abdominis muscle, in the absence of a previous surgical scar, is presented here.