A carboxyl or carboxylic group is a functional group consisting of a carbon atom doubly bonded to an oxygen atom and single-bonded to a hydroxyl (-OH) group. It is the univalent acid radical (-COOH or -CO₂H):

where R is a hydrogen or an organic group .

Carboxyl groups are characteristic constituents of more complex functional groups such as carboxylic acids and amides. They are present in most organic acids, this making them biodegradable. They are structurally similar to the carbonyl group.

The C=O stretch of carboxyl groups can be identified spectroscopically by a sharp peak in the infrared between 1700 and 2100 cm^-1.