Isobutyric acid, also known as 2-methylpropanoic acid, is a carboxylic acid with structural formula (CH₃)₂-CH-COOH. It is found in the free state in carobs (Ceratonia siliqua) and in the root of Arnica dulcis, and as an ethyl ester in croton oil.

Isobutyric acid is an isomer of n-butyric acid; they have the same chemical formula C₄H₈ O₂ but a different structure.

Isobutyric acid may be artificially prepared by the hydrolysis of isopropylcyanide with alkalies, by the oxidation of isopropyl alcohol with potassium bichromate and sulfuric acid (I. Pierre and E. Puchot, Ann. de chim. et de pliys., 1873, 28, p. 366), or by the action of sodium amalgam on methacrylic acid. It is a liquid of somewhat unpleasant smell, boiling at 155° C. Its specific gravity is 0.9697 (0°). Heated with chromic acid solution to 140° C., it gives carbon dioxide and acetone. Alkaline potassium permanganate oxidizes it to a-oxyisobutyric acid, (CH₃)₂-C(OH)-COOH, whilst concentrated nitric acid converts it into dinitroisopropanc . Its salts are more soluble in water than those of butyric acid.