CH₃Hg+ is a cation/functional group composed of a methyl group and a oxidized mercury -Hg(II)- atom. It also answers to the names monomethylmercury and MeHg.

It represents the most toxic form that mercury can adopt because -being an organic compound- it enters the biota, accumulates and biomagnifies in the food web, thus representing a threat for human health [4].

Not to be confused with CH₃HgCH₃ or dimethylmercury, which is also toxic and dangerous but it's mostly unreactive and does not bioaccumulates nor biomagnifies [3].