Adult emperor penguins Aptenodytes forsteri breed on fast ice and forage within sea ice in winter. However, it remains unknown whether juveniles exhibit similar foraging behavior during their early life at-sea movements, and how it links with the oceanographic conditions. We investigated the first at-sea odyssey of 15 juvenile emperor penguins from Terre Adélie in 2013-2014. The average tracking duration was 167 ± 110 d SD (range 86-344 d). After departing the colony in December/January, the juveniles traveled north up to 53.76°S before heading south in April/May to forage within the sea ice. The juveniles spent 49 ± 14% of their total recorded trips (n = 12) in the sea ice, over both the continental slope and deep ocean regions. The penguins dived primarily during daylight. Within sea ice, the juveniles performed both shallow and deep dives, with the proportion of each varying seasonally. The switch to primarily deep dives in the autumn and winter within sea ice may be a consequence of (1) a seasonal change in the krill distribution from surface to deep waters and/or (2) the presence of macrozooplankton at depth due to a reduced/absent diel migration. Furthermore, we showed for the first time that the diving behavior of juveniles was associated with the mixed layer depth. We suggest they feed on mesopelagic prey aggregating near the thermocline. This study provides insight into an important, but poorly understood, part of the emperor penguin life cycle, essential to predict their response to future climate change.