The water isotope composition of the winter snow cover on Arctic sea ice is strongly enriched in heavy isotopes near the snow-sea ice interface, incompatible with typical enrichment values through snow metamorphism processes alone. Our stratigraphic investigations from the MOSAiC expedition, using computed tomography combined with isotopic analyses of the snow, highlight that approximately 20% of the snowpack is not of meteoric origin but created from the sea ice. Here, we show that sea ice sublimation under the high-temperature gradients during the Arctic winter produces a snow-like structure and significantly contributes to the total snow water equivalent on Arctic sea ice. This, until now, unaccounted oceanographic source of “snow” furthers our understanding of i) vapor fluxes and gas exchange through the snowpack with biogeochemistry applications, ii) the formation of saline Arctic snow, and contribution to sea salt aerosols, iii) uncertainties in mass balance and physical properties of snow, and iv) additional uncertainties in precipitation estimates when compared to in situ measurements. Ultimately, regional differences in precipitation will result in varying local temperature gradients and therefore different contributions of ocean-sourced snow, and understanding this is essential for improving the accuracy of modeled sea ice predictions.