Paraldehyde is the cyclic form of three acetaldehyde molecules (a trimer). A colourless to pale yellow pungent liquid, it is sparingly soluble in water and highly soluble in alcohol.

It has been used medically as a CNS depressant, an anticonvulsant, or a hypnotic or sedative - especially to induce sleep in sufferers from delirium tremens. In the United States it is sold under the tradename Paral®, and is used orally or rectally - it can cause pulmonary oedema and haemorrhage if injected. Due to side-effects and a narrow safe dosage range it is rarely used today as a hypnotic, and is third or lower choice in seizure treatment. An overdose is marked by repiratory depression; lung, hepatic, or renal damage; coma and death. It can be addictive, despite its odour and unpleasant flavour.

It is also used in resin manufacture, as a preservative, and in other processes as a solvent. It reacts with plastic and so must be stored in glass. It can decompose in air, turning brown and producing an odour of acetic acid.

Its empirical formula is C₆H₁₂O₃, its molecular weight 132.16, it melts at 12.5 °C, boils at 124 °C and is very flammable with a flashpoint of 27.5 °C.

It has numerous alternate names including paracetaldehyde, s-trimethyltrioxymethylene, and trimethyl-1,3,5-trioxane.