The pathological and histopathological alterations associated with parasitic infection induced by the didymozoid Pozdnyakovia gibsoni Justo and Kohn, 2012, in the stomach wall of the skipjack tuna Katsuwonus pelamis (Linnaeus, 1758) in the southwestern Atlantic Ocean off South America are presented. The parasites were encysted in the mucosal layer of the stomach and inserted into the connective tissue that forms the lamina propria. The parasite penetrates through the stomach wall, inducing white blood cell proliferation and a localized inflammatory reaction.