In early February 2025, a 12-hour snowfall of 120 cm was observed in Obihiro, located on the Tokachi Plain of southeastern Hokkaido, Japan; it was the highest recorded snowfall in Japan. Concurrently, a marine heatwave (MHW) with pronounced warm sea surface temperature was observed offshore. While MHWs effect on rainfall are documented, their impact on snowfall remains poorly understood. Here, we demonstrated the mechanisms behind the record-breaking snowfall event, including the effects of the MHW. During the heavy snowfall, an extratropical cyclone drove strong easterly winds toward the coastal regions of the Tokachi Plain, and a surface front was located to the south of Obihiro. The easterly winds transported a convectively unstable layer from over the ocean into the front, and the frontal updrafts released the instability. Consequently, convective precipitation systems developed, yielding heavy snowfall at Obihiro on the cold side of the front. Notably, the MHW enhanced the frontal formation and convective instability, increasing precipitation around Obihiro by approximately 50%. This case study demonstrates that MHWs can significantly amplify snowfall under specific atmospheric conditions, advancing our understanding of compound extreme.