Indole-3-acetic acid (IAA, 3-IAA) is the most common, naturally occurring, plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. IAA is a derivative of indole, containing a carboxymethyl substituent. It is a colorless solid that is soluble in polar organic solvents.

IAA is predominantly produced in the apical bud of and young leaves of plants and is known to be an inducer of cell division and elongation. IAA, as well as most other auxins, are also known to act in concert with, or in opposition to, other plant hormones. For example, the ratio of auxin to cytokinin in certain plant tissues determines initiation of root versus shoot buds.

It is a relatively low auxin, as is AIB (indolbutyric acid β). It is a growth regulator often used in in vitro culture, and more particularly in certain physiological research because it has the advantage of being a natural substance. For the same reason, it is sensitive to auxin degradation enzyme systems, and its solutions lack stability and oxidize in light.