Syphacia (Syphacia) millardiae n. sp. (Nematoda: Oxyuridae: Syphaciinae) is described based on specimens collected from a soft-furred field rat, Millardia meltada. This species is closely related to several other members of the genus Syphacia, also murid parasites. This species closely resembles Syphacia (Syphacia) obvelata, a cosmopolitan parasite of Mus musculus. Syphacia (Syphacia) millardiae n. sp. differs from S. obvelata in that females exhibit a cuticular ornamentation with varicose cuticular elevations between the cephalic nervous endings, smaller body width, smaller eggs, and a longer esophagus. Males exhibit a shorter body length, a longer tail with a shorter tail tip, a longer esophagus, and a longer gubernaculum than do males of S. obvelata.