The phenyl group is a monovalent organic radical, derived from benzene. It has the chemical formula C₆H₅ and it is sometimes abbreviated to -Ph. The phenyl group is a modifier of organic functional groups. The phenyl group is basically a benzene ring with a hydrogen removed. The phenyl group is a ring structure, and as such, it is an aryl group (an organic group derived from an aromatic hydrocarbon with the absence of one hydrogen).

Similar to other organic modifier groups, the phenyl group alters the physical and chemical properties of the substance it bonds to. For example, the boiling point of a chemical can be increased by replacing a hydrogen atom with the phenyl group. In general as the molecular weight of a compound increases, the boiling point increases. The phenyl group is unsaturated, that is there are double bonds present that do not have the maximum number of hydrogens attached to them. As such, compounds containing the phenyl group are able to participate in all of the reactions of an unsaturated compound. They are able to undergo hydrogenation with hydrogen adding across the double bonds.Bromine water added to a compound with the phenyl group will decolorize. This is a standard test for unsaturated compounds.

The polymer polystyrene contains the phenyl group as part of its repeat unit. The organic chemical phenol could also be called phenyl hydroxide, it has the chemical formula C₆H₅OH.

Phenylalanine contains the phenyl group and is one of only three amino acids containing an aromatic group (a ring structure). As part of the metabolism of phenylalanine, oxygen is used to break the ring structure, and eventually after several intermediate steps, the aliphatic (straight chain) molecules fumarate and acetoacetate are produced. The absence of the functioning enzyme that catalyzes this reaction produces a disease called phenylketonuria, which is genetically controlled and leads to mental disorders.Aniline itself contains the phenyl group and its formula can be written as PhNH₂. Aniline is used in the manufacture of a number of products including dyes, pharmaceuticals, antioxidants, and vulcanization accelerators in rubber compounds.

The phenyl group is often encountered as an intermediate in many organic reactions. This is particularly common in reactions occurring in solution. The phenyl group by itself is unstable and, as such, will rapidly react with some other compound.