<p>The Antarctic deep-sea smelt (<i>Bathylagus antarcticus</i>) is one of the most abundant mesopelagic fishes in the Southern Ocean, however, knowledge of its diet is limited and its feeding habits remain poorly understood. Using 1,069 specimens collected during the austral summers of 2021 to 2023 in the Cosmonaut Sea, the morphological data from the gut contents of <i>B. antarcticus</i> was quantitatively studied to provide new information on feeding habits of the species, including its dietary variation with fish size, geographic location, and circadian rhythm. The results showed a diverse diet of <i>B. antarcticus</i> with mainly crustacean zooplankton, such as copepods, euphausiids, amphipods, and ostracods, and also of soft-bodied items, and the diet was found to be influenced by the species composition of the local zooplankton community. Moreover, a transition in dietary preference from copepods to euphausiids was identified during ontogeny, and the length of the fish undergoing the shifts roughly matched the thickening of the fish body. The species can be ascribed to, especially during twilight hours, adjust vertical migrations to actively pursue the most energetically profitable prey organisms in the water column, thereby optimizing their diet and sustaining the population. In conclusion, <i>B. antarcticus</i> exhibited considerable dietary specialization, with fish size identified as the primary determinant accounting for intraspecific resource partitioning. The dietary information presented here enhances our current understanding of the nutritional structure of this poorly known species and demonstrates that <i>B. antarcticus</i> contributes to Antarctic food webs through both krill-dependent and krill-independent trophic pathways.</p>