In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula  and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy groups. Both the negatively charged anion , called hydroxide, and the neutral radical , known as the hydroxyl radical, consist of an unbonded hydroxy group.
According to IUPAC definitions, the term hydroxyl refers to the hydroxyl radical () only, while the functional group  is called a hydroxy group.