Alkylmercury compounds having methyl groups. Includes both mono- and dimethylmercury.

In fact, methylmercury is not a compound in itself but a cation, CH₃Hg⁺, which forms one part of methylmercury compounds; usually methylmercury salts. Dimethylmercury is one methylmercury compound that is not a salt. The methylmercury cation is normally associated with either a simple anion, like chloride (Cl^-), or a large molecule (e.g. a protein) with negative and positive charges. The methylmercury cation is the most toxic form of mercury, able to inhibit fetal brain development, which results in the behavioural changes and reduced cognitive and motor ability.