This dataset presents the diet composition of tuna during autumn migrations to the Iceland Basin south of the continental shelf for the year 1998 and 1999 (area Va and XII defined by ICES). Each line in the dataset represents a different prey item found in the stomach of one tuna fish with a specific ID number. Therefore, several lines represent the total number of prey item found in one tuna stomach. For the stomach content analysis, the data were separated by geographical areas recorded in the column G called “area”. Tuna were also classified according to their size distribution defined in the column “groups”. A total of 36 prey species or higher taxa are described in 421 stomach samples: 17 teleost fishes, 4 squid, 1 octopus, 11 crustaceans and 2 other invertebrate species. The most important prey species are European flying squid (Todarodes sagittatus), barracudinas (Paralepididae) and gonate squid (Gonatus sp.). The identification of prey items was performed by identification of individual prey items, and of sagittal otolith and of beak pairs for fish and squid respectively, when preys were heavily digested.