Umbilical endometrioma is a rare condition, with an estimated incidence of 0.5%-1% in all patients with endometrial ectopia. Spontaneous abdominal wall endometriosis is an even rarer and more unusual condition with unclear pathogenetic mechanisms. A 44-year-old parous woman presented with an umbilical skin lesion, and no history of bleeding from the umbilical mass or swelling in the umbilical area. The initial clinical diagnosis was granuloma, and excision was planned. Pathology examination revealed endometrial glands with mucinous-type metaplasia surrounded by a disintegrating mantle of endometrial stroma. Clinical examination and magnetic resonance imaging did not reveal pelvic endometriosis lesions, and given that the umbilical endometrioma was totally excised, no further treatment with hormonal therapy was proposed for the patient. Three years after excision, she was free of disease and no recurrence has been observed. Complete excision and histology is highly recommended for obtaining a definitive diagnosis and optimal treatment in spontaneous abdominal wall endometriosis.