Four alien monogeneans, Unilatus unilatus Mizelle and Kritsky, 1967, U. brittani Mizelle, Kritsky, and Crane, 1968, Trinigyrus peregrinus n. sp., and Heteropriapulus heterotylus (Jogunoori, Kritsky, and Venkatanarasaiah, 2004), were collected from the gills of vermiculated sailfin catfish, Pterygoplichthys disjunctivus (Weber, 1991) (Siluriformes: Loricariidae), in inland waters of Okinawa-jima island, Okinawa Prefecture, Japan. Unilatus unilatus and U. brittani represent new country records for Japan. Trinigyrus peregrinus n. sp. is characterized by a coiled male copulatory organ that forms a circle and does not articulate with its accessory piece. Heteropriapulus heterotylus was newly collected from two rivers in south-central Okinawa-jima island. These monogeneans are all considered to be native to South America and to have been co-introduced with the host fish into the inland waters of the island by release of ornamental pet fish.