硬脂酸镁，分子式(C₁₇H₃₅COO)₂Mg。

白色无臭无味细软光亮粉末。微溶于水，溶于热的乙醇溶液。与强酸反应生成硬脂酸和相应的镁盐。工业品含少量油酸和7％的氧化镁。低毒。

Magnesium stearate, also called octadecanoic acid, magnesium salt, is a white substance, powder which becomes solid at room temperature. It has the chemical formula Mg(C
18H
35O
2)
2. It is a salt containing two equivalents of stearate (the anion of stearic acid) and one magnesium cation (Mg). Magnesium stearate melts at about 88 °C, is not soluble in water, and is generally considered safe for human consumption at levels below 2500 mg/kg per day. In 1979, the FDA's Subcommittee on GRAS (generally recognized as safe) Substances (SCOGS) reported, "There is no evidence in the available information on ... magnesium stearate ... that demonstrates, or suggests reasonable grounds to suspect, a hazard to the public when they are used at levels that are now current and in the manner now practiced, or which might reasonably be expected in the future."