磺酸是含有磺酸基-SO₂OH（-S(=O)(=O)-OH）的一类有机化合物。一般它们的酸性比相应羧酸强，可与蛋白质和碳水化合物紧密结合。在很多反应中用作催化剂和反应中间体。

磺酸基与氢相连形成的H-SO₂-OH很不稳定，很快经互变异构转变为亚硫酸HO-S(=O)-OH。

清洁剂、抗细菌药物（磺胺）、阴离子交换树脂和染料大多含有磺酸或磺酸盐结构。

常见的有机磺酸包括：

A sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula R−S(=O)₂−OH, where R is an organic alkyl or aryl group and the S(=O)₂−OH group a sulfonyl hydroxide. A sulfonic acid can be thought of as sulfuric acid with one hydroxyl group replaced by an organic substituent. The parent compound (with the organic substituent replaced by hydrogen) is the hypothetical compound sulfurous acid. Salts or esters of sulfonic acids are called sulfonates.