In chemistry, a carbonyl group is a functional group composed of an atom of carbon double-bonded to an atom of oxygen. It is the functional group present in aldehydes (where it is located terminally) and ketones (internally), and forms the basis for several other functional groups, such as those of carboxylic acids (alcohol-branched carboxyl group) and amides (NH-branched).

The carbon-oxygen double bond absorbs infrared electromagnetic radiation at wavenumbers between approximately 1760–1665 cm^-1. In IR spectroscopy, such characteristic absorption is commonly known as the "carbonyl stretch" and is present in ketones, aldehydes, esters, carboxylic acids, and other carbonyl-containing compounds.