<p>The mesopelagic zone increasingly attracts the interest of commercial fisheries due to its large fish abundance and the growing worldwide demand for food. Many mesopelagic species serve as an important food source for many top predators but the extent to which these predators rely on mesopelagic prey remain poorly understood, especially in oligotrophic open ocean ecosystems. The Azores archipelago is surrounded by numerous seamounts and is subject to a dynamic oceanography, creating particularly attractive conditions for many top predators, including cetaceans. Using stable isotope mixing models (MixSIAR), we estimated diet proportions of seven species of cetaceans belonging to various trophic guilds. Overall, mesopelagic prey are key contributors to the diet of cetaceans. We find that <i>Balaenoptera borealis</i>, <i>B. musculus </i>and <i>B. physalus </i>consume mostly (&gt; 70%) mesopelagic zooplankton (including copepods and krill). Mesopelagic fish and cephalopods contribute 20–35% to the diet of <i>Delphinus delphis</i>, <i>Stenella frontalis</i> and <i>Tursiops truncatus</i>, along with epipelagic and demersal fish. The diet of <i>Physeter macrocephalus </i>almost exclusively consists of mesopelagic cephalopods (&gt; 94%). This study identifies mesopelagic fauna as a critical food resource for cetaceans in tropical and sub-tropical oceanic ecosystems, emphasizing the need for a precautionary approach and strict governance measures before developing commercial fisheries in the mesopelagic zone.</p>