Two hundred and forty-four specimens of Parapharyngodon riojensis n. sp. were found in the large intestines of 2 adult lizards Phymaturus punae collected from Quebrada del Leoncito, Province of La Rioja, Argentina. Parapharyngodon riojensis n. sp. represents the ninth species of the genus from the Neotropical Realm and the first species to be described from Argentina. It can be distinguished from all species of Parapharyngodon on the basis of the morphology of the anterior cloacal lip, the location of the ovary, and geographical distribution. Parapharyngodon riojensis n. sp. is most similar to P. senisfaciecaudus in that the ovary does coil around the esophagus and the number and location of caudal papillae in the males are the same. These 2 species differ in that the eggs of P. senisfaciecaudus are slightly asymmetrical, with a smooth, thin shell, whereas the eggs of P. riojensis are oval, with a punctate thick shell. In addition, the cloacal lip of males of P. senisfaciecaudus is smooth, whereas the cloacal lip of males of P. riojensis is echinate. A key to the species of Parapharyngodon in the Neotropical Realm is provided.