In organic chemistry, a keto-enol tautomerism is an equilibrium between a ketone or aldehyde (the keto form) and an enol. The interconversion of the two forms involves only the movement of a proton and the shifting of bonding electrons, and hence the isomerism qualifies as a tautomerism.

A compound containing a carbonyl group, C=O, is normally in rapid equilibrium with an enol isomer, which contains a pair of double bonded carbon atoms adjacent to an OH or alcohol group, C=C-OH. The keto form predominates at equilibrium for most ketones and aldehydes (certain aromatic compounds such as phenol are exceptions, due to the aromatic character of the enol but not the keto form). Nonetheless, the enol form is important for some reactions of the compound. Furthermore, the deprotonated intermediate in the interconversion of the two forms, referred to as an enolate anion, is important in carbonyl chemistry, in large part because it is a good nucleophile.

Keto-enol tautomerism is important in several areas of biochemistry. The high phosphate-transfer potential of phosphoenolpyruvate results from the fact that the phosphorylated compound is "trapped" in the less stable enol form, whereas after dephosphorylation it can assume the keto form. Rare enol tautomers of the bases guanine and thymine can lead to mutation because of their altered base-pairing properties.