Sub‐mesoscale fronts—with scales from 1 to 50 km are ubiquitous in satellite images of the world oceans. They are known to generate strong vertical velocities with significant impacts on biogeochemical fluxes and pelagic ecosystems. Here, we use a unique data set, combining high‐resolution behavioral and physical measurements, to determine the effects of sub‐mesoscale structures on the foraging behavior of 12 instrumented female southern elephant seals. These marine mammals make long voyages (several months over more than 2000 km), diving and feeding continuously in the Antarctic Circumpolar Current. Our results show that elephant seals change their foraging behavior when crossing sub‐mesoscale fronts: They forage more and at shallower depths inside sub‐mesoscale fronts compared to nonfrontal areas, and they also reduce their horizontal velocity likely to concentrate on their vertical diving activity. The results highlight the importance of sub‐mesoscale fronts in enhancing prey accessibility for upper trophic levels, and suggest that trophic interactions are stimulated in these structures.