Human-induced expansion of oxygen-deficient zones can have dramatic impacts on marine systems and itsresident biota. One example is the formation of the potent neurotoxic methylmercury (MeHg) that is mediated bymicrobial methylation of inorganic divalent Hg (HgII) under oxygen-deficient conditions. A negative consequenceof the expansion of oxygen-deficient zones could be an increase in MeHg production due to shifts in microbialcommunities in favor of microorganisms methylating Hg. There is, however, limited knowledge about Hg-methylatingmicrobes, i.e., those carrying hgc genes critical for mediating the process, from marine sediments. Here, weaim to study the presence of hgc genes and transcripts in metagenomes and metatranscriptomes from four surfacesediments with contrasting concentrations of oxygen and sulfide in the Baltic Sea. We show that potential Hgmethylators differed among sediments depending on redox conditions. Sediments with an oxygenated surface featuredhgc-like genes and transcripts predominantly associated with uncultured Desulfobacterota (OalgD group)and Desulfobacterales (including Desulfobacula sp.) while sediments with a hypoxic-anoxic surface included hgccarryingVerrucomicrobia, unclassified Desulfobacterales, Desulfatiglandales, and uncharacterized microbes. Ourdata suggest that the expansion of oxygen-deficient zones in marine systems may lead to a compositional changeof Hg-methylating microbial groups in the sediments, where Hg methylators whose metabolism and biology havenot yet been characterized will be promoted and expand.