Dimethylmercury is an extremely toxic organomercury compound with the formula (CH₃)₂Hg. A volatile, flammable, dense and colorless liquid, dimethylmercury is one of the strongest known neurotoxins. Less than 0.1 mL is capable of inducing severe mercury poisoning resulting in death.

The compound was one of the earliest organometallics reported, reflecting its considerable stability. The compound was first prepared by George Buckton in 1857 by a reaction of methylmercury iodide with potassium cyanide:

Later, Edward Frankland discovered that it could be synthesized by treating sodium amalgam with methyl halides:

It can also be obtained by alkylation of mercuric chloride with methyllithium:

The molecule adopts a linear structure with Hg–C bond lengths of 2.083 Å.

Dimethylmercury is stable in water and reacts with mineral acids at a significant rate only at elevated temperatures, whereas the corresponding organocadmium and organozinc compounds (and most metal alkyls in general) hydrolyze rapidly. The difference reflects the high electronegativity of Hg (Pauling EN = 2.00) and the low affinity of Hg(II) for oxygen ligands. The compound undergoes a redistribution reaction with mercuric chloride to give methylmercury chloride:

Whereas dimethylmercury is a volatile liquid, methylmercury chloride is a crystalline solid.

Dimethylmercury has few practical applications because of its danger to human life. It has been studied for reactions involving bonding methylmercury cations to target molecules, forming potent bactericides, but methylmercury's bioaccumulation and ultimate toxicity has led to it being largely abandoned in favor of the less toxic ethylmercury and diethylmercury compounds, which perform a similar function without the bioaccumulation hazard.^{[citation needed]}