Thujone is a ketone and a monoterpene that occurs naturally in two diastereomeric forms: (−)-α-thujone and (+)-β-thujone. It has a menthol odor. Even though it is best known as a chemical compound in the spirit absinthe, recent tests show absinthe contains only small quantities of thujone, and it may or may not be responsible for absinthe's reported psychedelic effects. Thujone acts on GABA and 5-HT₃ receptors in the brain. In many countries the amount of thujone allowed in food or drink products is regulated.

In addition to the naturally occurring (−)-α-thujone and (+)-β-thujone, there are two other enantiomeric forms possible; (+)-α-thujone and (−)-β-thujone:

(−)-α-thujone

(+)-α-thujone

(+)-β-thujone

(−)-β-thujone

Thujone is found in a number of plants, such as arborvitae (genus Thuja, hence the derivation of the name), Nootka Cypress, some junipers, mugwort, oregano, common sage, tansy and wormwood, most notably grand wormwood (Artemisia absinthium), usually as a mix of isomers in a 1:2 ratio. It is also found in various species of mentha (mint).