The Canal of Nuck is the portion of the processus vaginalis within the inguinal canal in women. A hydrocele of the Canal of Nuck is equivalent to an encysted hydrocele of the spermatic cord in men. Little about this rare condition in the adult female patient has been reported in the literature. In this paper, we report on a case of hydrocele of the Canal of Nuck in a 30-year-old female. The diagnosis was made based on sonography and CT, and then confirmed during surgery. Although rare, a hydrocele of the Canal of Nuck must be included in the differential diagnosis of a groin lump in female patients.