Galaxitaenia n. gen. (Eucestoda: Pseudophyllidea) is proposed to accommodate Galaxitaenia toloi n. gen., n. sp. from Galaxias platei, a freshwater fish inhabiting Andean lakes in Argentine Patagonia. Galaxitaenia belongs to the Triaenophoridae because it possesses a marginal genital pore, a ventral uterine pore, and a follicular vitelline gland. The new genus can be distinguished from other triaenophorids by the following combination of characters: a scolex with a prominent rectangular apical disc without hooks, grooves, or indentations; neck present; segments wider than long to quadrangular; testes in 2 lateral fields, often connected anteriorly; cirrus unarmed; an ovary situated posteriorly; a vagina posterior to the cirrus sac; vitelline follicles medullary, in 2 ventral fields forming 2 wings interrupted medially; a uterus saccate to branched; and eggs without operculum. The types, distribution, and density of microtriches were analyzed on the surface of the tegument of scolex, neck, and immature segments. The presence of tumuli were observed on the apical disc of the scolex. Galaxitaenia toloi n. sp. is the first triaenophorid species recorded in a South American freshwater fish and the fourth helminth parasitizing the intestine of G. platei.