Monodelphoxyuris dollmeiri n. g., n. sp. is described from the caecum of Monodelphis emiliae (Thomas) (Marsupiala: Monodelphidae) collected in the eastern region of the Andes of Peru. M. dollmeiri n. sp. differs from the closest pinworm described from marsupials in the Neotropics (Didelphoxyuris thylamisis Gardner & Hugot, 1995) in having operculate eggs and males that possess a tip prolonged of tail and a different kind of area rugosa. Several other species of pinworms have been described from marsupials in Australia, but all are characterised by possessing of a buccal capsule that is strongly cuticularised with interradial lamellae. These structures are lacking both in M. dollmeiri and D. thylamisis. Monodelphoxyuris n. g. is characterised by a mouth opening into a depression and lateral alae composed of two longitudinal crests. Males possess an area rugosa composed of six ventral sagittal mamelons, a caudal extremity prolonged by a robust tip of the tail which is directed backwards, four pairs of genital papillae (two pairs lateral adanal and sessile, one pair just posterior to the spicule aperture, and the last pair at posterior extremity and pedunculate). Females possess an opisthodelphic uterus, a reflected ovary and operculate, unembryonated, oval eggs.