Oxindole (2-indolone) is an aromatic heterocyclic organic compound with the formula C₆H₄CH₂C(O)NH. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing ring. Oxindole is a modified indoline with a substituted carbonyl at the second position of the 5-member indoline ring. Classified as a cyclic amide, it is a pale yellow solid.

Oxindole is derived in nature from tryptophan, formed by gut bacteria ("normal flora"). It is normally metabolized and detoxified from the body by the liver. In excess, it can cause sedation, muscle weakness, hypotension, and coma. Patients with hepatic encephalopathy have been recorded to have elevated serum oxindole levels.

Treatment with phosphorus pentasulfide gives the thione.

Beyond the parent compound, the oxindole structure is present in many compounds including, for example, rhynchophylline, 3-methyloxindole, and semaxanib.