This article provides the first data on the occurrence of adult proteocephalidean tapeworms (Cestoda) in freshwater fishes from the Peruvian part of the Amazon River. In total, 29 species of proteocephalidean cestodes of the genera Amphoteromorphus, Chambriella, Choanoscolex, Euzetiella, Houssayela, Mariauxiella, Monticellia, Nomimoscolex, Nupelia, Peltidocotyle, Proteocephalus, Rudolphiella, Scholzia, Sciadocephalus, Spatulifer, Travassiella, and Zygobothrium are reported from 10 species of pimelodid, heptapterid and doradid catfish and the cichlids Astronotus ocellatus and Cichla monoculus. All species are reported from western Amazonia and Peru for the first time, with 6 taxa representing new geographical records for the Amazon River basin. The most remarkable finding is that of Houssayela sudobim (Woodland, 1935) from Pseudoplatystoma fasciatus, reported for the first time since its original description. Four other taxa (Chambriella sp. and Proteocephalus sp. from Phractocephalus hemioliopterus, Nupelia sp. from Goeldiella eques, and Proteocephalus sp. from Pterodoras granulosus) may be new for science. A list of the species found is provided together with brief comments on their fish hosts and geographical distribution. In addition to adult tapeworms, larvae (metacestodes) of proteocephalideans were found encysted in the mesenteries, intestinal wall, and gills of small catfish.