<jats:title>Abstract</jats:title> <jats:p>Supercooled water (SCW) with temperature below the in situ freezing point is a ubiquitous phenomenon accompanied by frazil and platelet ice production in polar waters. Despite its significance, a thorough understanding of SCW beneath the Arctic sea ice, particularly regarding its seasonal evolution, spatial variation, and influencing factors, is missing. Using hydrographic data collected by autonomous ice‐tethered buoys, including the Drift‐towing Oceanic Profiler and Ice‐Tethered Profiler, we describe the characteristics of a widespread several‐meter‐thick SCW layer underneath the ice, which strengthens from mid‐fall to winter and weakens in spring and is closely related to the seasonal evolution of near‐surface air temperature. Significant regional differences in supercooling characteristics were observed across the Arctic Ocean. Supercooling is most pronounced in the Eastern Central Arctic, with average supercooling magnitudes of 5–8 mK (uncertainty: ±3 mK) underneath the ice and proportions of supercooled profiles approaching 100% during winter. In contrast, the Beaufort Gyre region exhibits the weakest supercooling and greater spatial variability due to enhanced subsurface heat entrainment induced by storms.</jats:p>