Chloral hydrate is a sedative and hypnotic drug, also known as trichloroacetaldehyde monohydrate, 2,2,2-trichloro-1,1-ethanediol, and the tradenames Aquachloral, Novo-Chlorhydrate, Somnos, and Noctec. Its chemical formula is ₃₃₂.

It is used for the short-term treatment of insomnia and as a sedative before minor medical or dental treatment. It has been largely displaced by the development of benzodiazepines. It is also used as in veterinary medicine as a general anesthetic.

In therapeutic doses for insomnia it is effective within sixty minutes, it is rapidly metabolized into trichloroethanol and trichloroacetic acid. Higher doses can depress respiration and blood pressure. An overdose is marked by confusion, convulsions, nausea and vomiting, severe drowsiness, slow and irregular breathing, cardiac arrhythmia and weakness. It may also cause liver damage. It is moderately addictive. Chronic use can cause dependency and withdrawal symptoms. It can potentiate various anticoagulants and is weakly mutagenic in vitro and in vivo.

It was discovered through the chlorination of ethanol in 1832 by Justus von Liebig in Giessen. It was widely abused and misprescribed in the late 19th century.

Mixed with alcohol it was the first "Mickey Finn".

It is a minor side-product of the chlorination of water; concentrations rarely exceed 5 micrograms per litre (µg/l).

The anhydrous form is an intermediate in insecticide and herbicide manufacture (including DDT, dichlorvos , and naled ).

Chloral hydrate is a schedule IV controlled substance in the United States.