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An aryl group, or aryl substituent, is an aromatic ring system attached to another molecule via one carbon atom, formed by removing a hydrogen atom from an arene. In structural formulas it is often denoted as Ar−. The most common aryl group is the phenyl group, derived from benzene, with the formula C6H5−. Other examples include naphthyl (from naphthalene) and more complex arenes used as substituents.

Aryl groups are typically planar and sp2-hybridized, reflecting the rigid, conjugated nature of the aromatic ring. They are connected to the rest of a molecule through the aryl carbon that bears the site of attachment. The electronic effects of aryl groups depend on substituents; they can donate or withdraw electrons via resonance and inductive effects, influencing reactivity and properties. Aryl groups contrast with alkyl groups, which are usually sp3 and more flexible.

In naming, aryl substituents are described by names such as phenyl, tolyl, or chlorophenyl, with the suffix -yl indicating a removed hydrogen. The phenyl group is the simplest and most widely encountered aryl group. Aryl groups are central to many reactions, especially cross-coupling techniques that form carbon–carbon bonds, such as Suzuki–Miyaura, Heck, and Sonogashira couplings, used to install aryl rings onto various substrates.

Applications of aryl groups include pharmaceuticals, dyes, agrochemicals, and polymer science. They influence properties such as rigidity, planarity, metabolic stability, and intermolecular interactions. Understanding aryl groups is fundamental in organic synthesis and molecular design.