Indole-3-butyric acid (1H-indole-3-butanoic acid, IBA) is a plant hormone in the auxin family and is an ingredient in many commercial horticultural plant rooting products.

Aside from accelerating root formation, it is used on various crops to stimulate flower development and the growth of fruits. This ultimately increases crop yields.

Historically, products that contained IBA were used to protect plants during transplantation by stimulating root growth and decreasing shock.

In plant tissue culture IBA and other auxins are used to initiate root formation in vitro in a procedure called micropropagation. Micropropagation of plants is the process of using small samples of plants called explants and causing them to undergo growth of differentiated or undifferentiated cells. In connection with cytokinins like kinetin, auxins like IBA can be used to cause the formation of masses of undifferentiated cells called callus. Callus formation is often used as a first step process in micropropagation where the callus cells are then caused to form other tissues such as roots by exposing them to certain hormones like auxins that produce roots.