We present the first case of primary vulvar mucinous adenocarcinoma of ectopic breast origin. The patient is an 84-year-old woman with a mass on the left side of her vulva. A left partial vulvectomy with bilateral inguinal lymph node dissections revealed a mucinous adenocarcinoma that involved the dermis and subcutaneous tissue. The tumor cells were positive for estrogen receptors (ERs), progesterone receptors (PRs), and BRST-1 markers. The clinical and pathologic features, differential diagnosis, and treatment are discussed.