Acanthochondria lilianae n. sp. (Copepoda: Chondracanthidae) is described and illustrated based on specimens of both sexes collected from inner surface of the operculum of the longtail southern cod, Patagonotothen ramsayi (Regan) (Perciformes: Nototheniidae), from the Patagonian Shelf, Argentina (42–48°S, 60–63°W). Acanthochondria lilianae n. sp. is characterized by the combination of a Type B-III antennule and Type A leg 2, in addition to both the cephalosome and the trunk being wider than long. The new species most closely resembles Acanthochondria incisa Shiino, 1955, Acanthochondria ophidii (Krøyer, 1863), Acanthochondria priacanthi Shiino, 1964, and Acanthochondria tasmaniae Heegaard, 1962, but differs from these species in the general measurements and proportions of the body, shape and size of head, shape and size of genitoabdomen, and fine details of appendages such as the armature of antennule, ornamentation of both pairs of legs, number of teeth on mandible and maxilla, and ornamentation on maxilliped. This is the first Acanthochondria species recorded from a nototheniid and the second southernmost record of a species of this genus in the southwestern Atlantic Ocean.