Oxindole (2-indolone) is an aromatic heterocyclic organic compound with the formula C6H4CH2C(O)NH.

Classified as a cyclic amide, it is a pale yellow solid.

Oxindole is derived in nature from tryptophan, formed by gut bacteria ("normal flora").

In excess, it can cause sedation, muscle weakness, hypotension, and coma.

Patients with hepatic encephalopathy have been recorded to have elevated serum oxindole levels.