A polyprotic acid is an acid that contains several ionisable hydrogen atoms and so can donate more than one proton (H⁺) per molecule, usually in successive steps. Each proton is released with a different ionisation constant, so the protons are lost one stage at a time. Sulfuric acid (H₂SO₄) is a polyprotic acid, in contrast to a monoprotic acid such as HCl.

多元酸是含有多个可电离氢原子的酸,因而每个分子可给出不止一个质子(H⁺),通常分步进行。每个质子以不同的电离常数释放,故各质子逐级失去。硫酸(H₂SO₄)是多元酸;与之相对,盐酸(HCl)等是一元酸。