An emended diagnosis of Thaumatocotyle Scott, 1904 (Monogenoidea: Monocotylidae) and a list of its species are provided. Merizocotyle urolophi Chisholm and Whittington, 1999, Merizocotyle macrostrobus Chisholm and Whittington, 2012, Merizocotyle papillae Chisholm and Whittington, 2012, and Merizocotyle rhadinopeos Chisholm and Whittington, 2012 are transferred to Thaumatocotyle as new combinations. Thaumatocotyle pseudodasybatis Hargis, 1955 is redescribed and is currently restricted to Thaumatocotyle-like helminths parasitizing the olfactory organs of the white-spotted eagle ray Aetobatus narinari (Euphrasen, 1790) (Myliobatiformes: Aetobatidae) in the western Atlantic Ocean. Two new species are described that infect the olfactory organs of pelagic eagle rays: Thaumatocotyle adelpha n. sp. from the ocellated eagle ray Aetobatus ocellatus (Kuhl, 1823) in the central and western Pacific Ocean and Thaumatocotyle casigneta n. sp. from the Pacific eagle ray Aetobatus laticeps Gill, 1865 from the eastern Pacific Ocean off the western coast of Mexico. Based on comparative morphology, T. pseudodasybatis, T. adelpha, and T. casigneta appear to form a subgroup of species that developed secondarily within the genus.