Stearic acid is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a waxy solid and its chemical formula is C₁₇H₃₅CO₂H. Its name comes from the Greek word στέαρ "stéar", which means tallow. The salts and esters of stearic acid are called stearates.

Stearic acid is used mainly in the manufacture of soaps, detergents, and several other cosmetics such as shaving creams and shampoos. Soaps are not produced directly from this compound, but indirectly via the saponification of stearic acid esters consisting of triglycerides. Stearic acid esters of ethylene glycol, glycol stearate, and glycol distearate are used in shampoos, soaps, and other daily use cosmetic items to achieve a pearly effect.