The most frequent etiologic agent of diphyllobothriosis in South America and the only one confirmed by molecular data in human cases in Peru is Adenocephalus pacificus (syn. Diphyllobothrium pacificum). This cestode is transmitted by ingestion of the plerocercoids found in marine fish, causing a parasitic zoonosis. The objective of the present study was to identify two cestodes isolated from two specimens of the South American sea lion (Otaria byronia) stranded on the beaches of Huacho and Barranca cities, located on the northern Peruvian coasts, in the department of Lima. Tapeworms were confirmed by morphological characteristics due to the presence of transverse papilla-like tegumental protuberances in proglottids and small sized eggs, as well as by sequencing of the partial cytochrome c oxidase subunit 1 (mtDNA-COI) gene that are congruent with additional available A. pacificus sequences. Even though sea lions in Peru are distributed along the coast and in areas of difficult access, generally located in protected natural areas, the fortuitous finding represented an opportunity to confirm the presence of A. pacificus in South American sea lions. This report of tapeworm A. pacificus could allow future monitoring of the occurrence and geographical distribution of this causative agent in epidemiological studies, since it is one of the main species of zoonotic importance in Peru.