Understanding snow processes in the sea-ice system is essential to improving Arctic sea-ice predictions and climate modeling. We show that the winter snow cover on Arctic sea ice is strongly enriched in heavy isotopes near the snow-sea ice interface, unexplainable by snow metamorphism alone. During the MOSAiC expedition, stratigraphic investigations revealed that large temperature gradients drive water vapor transport and mass transfer from sea ice into the snowpack. We estimate the contributed snow depth equivalent as 39 ± 7 mm (cumulative mass redistribution) and 63 ± 21 mm (isotope two-source model). Despite uncertainties, both highlight the need for detailed snowpack vapor flux modeling. Recognizing this recrystallization process improves understanding of snow stratigraphy, gas exchange, atmospheric chemistry through snow impurity distributions (e.g., sea salt aerosol), and reduces uncertainties in snow mass balance and heat conductivity. With continued Arctic change, evolving snowpack temperature gradients and recrystallized sea-ice snow contributions will further shape these processes.