These data are part of the Gulf Watch Alaska (GWA) long-term monitoring program and contain diet data from Black-legged Kittiwakes (BLKI), Rhinoceros Auklets (RHAU), and Tufted puffins (TUPU) located on Middleton Island in Prince William Sound, Gulf of Alaska. The first table consists of information on seabird diet samples including the sampling method and number of prey types identified per sample ("seabirdDiets_foodSample_middletonIsland.csv"). The second table is similar to the first table, however, the samples have been subdivided by individual prey species ("seabirdDiets_samplePrey_middletonIsland.csv"). The third table consists of information specific to sampled prey items, such as prey taxa identification and morphometrics ("seabirdDiets_preyItems_middletonIsland.csv"). The fourth table consists of processed diet information and indices specific to Black-legged Kittiwake and Rhinoceros Auklet prey ("seabirdDiets_RHAU_BLKI_diets_middletonIsland.csv"). The fifth table contains prey species and seabird species taxonomy information ("seabirdDiets_speciesTaxonomy_middletonIsland.csv"). All samples included in the tables of this data release were collected during the spring and summer months from 1978-2022.