Thaumasioscolex didelphidis n. gen., n. sp. is described from the intestine of the black-eared opossum Didelphis marsupialis L. (Marsupialia: Didelphidae) from Los Tuxtlas, Veracruz, Mexico. The new genus differs from all proteocephalidean genera in the morphology of the scolex that is formed by 4 well separated lobes each containing 1 noncircular sucker opening laterally inside the exterolateral cavity, a large-sized body (length up to 1 m), a large number of testes, the shape of gravid proglottids that are inversely craspedote (the anterior border of a proglottid overlaps the posterior border of a preceding proglottid), eggs in groups mostly of 4-6 eggs each, and an embryophore bearing digitiform projections on its external surface. This is the first tapeworm of the Proteocephalidea, the members of which were previously reported exclusively from poikilotherm vertebrates (freshwater fishes, amphibians, and reptiles), found in a homoiotherm vertebrate.