Stearic acid, also called octadecanoic acid, is one of the useful types of saturated fatty acids that comes from many animal and vegetable fats and oils. It is a waxy solid, and its chemical formula is CH₃(CH₂)₁₆COOH. Its name comes from the Greek word, stear, which means tallow. Its IUPAC name is octadecanoic acid.

Stearic acid is prepared by treating animal fat with water at a high pressure and temperature. It can also be obtained from the hydrogenation  of vegetable oils. Common stearic acid is actually a mix of stearic acid and palmitic acid, although purified stearic acid is available separately.

Stearic acid is useful as an ingredient in making candles, soaps, and cosmestics and for softening rubber.