In organic chemistry, a lactol is the cyclic equivalent of a hemiacetal or a hemiketal. The compound is formed by the intramolecular nucleophilic addition of a hydroxyl group to the carbonyl group of an aldehyde or a ketone. 
The lactol functional group is prevalent in nature as component of aldose sugars.
Lactols can participate in a variety of chemical reactions including:
- Oxidation to form lactones
- Reaction with alcohols to form acetals
- Reduction (deoxygenation) to form cyclic ethers
- IUPAC Gold Book lactols
- Lundt, Inge (2001). "Oxidation, reduction and deoxygenation of carbohydrates". Glycoscience (1): 501–531.