The extent of submarine permafrost on the Arctic shelves has been declining in recent decades due to global climate change, however, the influence of submarine permafrost thawing on methylmercury (MeHg) production in surface sediment has been understudied to date. In this context, the total Hg (THg) and MeHg concentrations and conventional sediment properties in the shelf and slope sites of the East Siberian Sea were observed to identify the seasonal permafrost thawing effect on the methylation of Hg(II). Top (0–2 cm) sediment was collected from the 12 sites on the East Siberian Shelf and Slope on the icebreaker R/V Araon in August 2019. THg in surface sediment was higher in the slope than in shelf sites due to the hydrodynamic sorting of fine particles enriched with Hg and Mn/Fe oxides. In contrast, the highest MeHg/THg ratios in surface sediment were noted at 50–60 m isobaths of the shelf, identified as a continuous-discontinuous transition zone, with greater fractions of photoreactive humic in the pore water and organic sulfur content in sediment than in the surrounding sites. This suggests that a copious supply of dissolved organic matter in the active layer of the permafrost transition zone promotes the in situ methylation rate of Hg(II). To confirm this hypothesis in the Beaufort Sea with the field-measured Hg(II) methylation rate constant (k_m), thirteen 40–50-cm-long sediment cores were collected from August to September 2022 on the R/V Araon. We are currently analyzing concentrations of THg and MeHg, as well as k_m values in each depth of these sediment cores. The effects of seasonal submarine permafrost thawing on the MeHg production rate will be identified when the experimental analysis is complete.