Background and Clinical Significance: Abdominal wall endometriosis (AWE) is an ectopic endometrial tissue embedded into the anterior abdominal wall, mainly infiltrating the rectus abdominis or oblique muscles and subcutaneous tissue. In most cases, AWE is associated with surgical scars after obstetrical and gynecological, as well as non-gynecological surgeries. Case Presentation: A 51-year-old female patient presented to the ultrasound outpatient clinic with a non-cyclic painful, palpable nodular mass located in the postoperative scar in the right lower abdominal quadrant. She underwent an appendectomy at the age of 9 (premenarchal period). The patient had regular menstrual cycles, one cesarean section and two vaginal deliveries, denied any trauma to that abdominal region, and had no history of pelvic endometriosis. Her past medical history was also remarkable for left-sided breast cancer, and she was worried it could be metastasis. Following imaging evaluation, a preliminary diagnosis was a benign lesion in the post-appendectomy scar, most likely a suture granuloma, also known as Schloffer’s tumor. Fine-needle aspiration was performed, and findings were primarily suspicious for AWE. The patient was then referred to an abdominal surgeon for excision of the affected area, and subsequent histopathological analysis confirmed that the mass was AWE. Conclusions: Imaging findings of a mass in the abdominal wall are not pathognomonic for AWE; only histopathological examination can confirm the diagnosis. If a painful nodular mass is located adjacent to a surgical scar in a female patient, AWE should be a leading consideration in the differential diagnosis, along with suture granuloma in cases of old surgical scars.