羧酸是带有羧基的有机化合物，通式是R-COOH。羧基的化学式为-C(=O)OH，通常写成-COOH或-CO₂H，它是由一个羰基（RR'C=O）和一个羟基（R-O-H）组成的官能基。羧酸是质子（H）授予者，也就是质子酸。

蚂蚁和蜜蜂的分泌液中含的甲酸H-COOH、使醋有酸味的乙酸CH₃-COOH……等都是常见的羧酸。拥有两个羧基的分子叫二酸，最简单的分子是由两个羧基结合而成的草酸(COOH)₂。其他像是可在柠檬中发现的柠檬酸和酸豆内含的酒石酸都是自然界中重要的羧酸。

羧基去质子化以后的共轭碱便是含有羧酸根负离子（-COO）的羧酸盐，与醇类混合经酯化反应后形成羧酸酯。羧酸离子因为共振而比较稳定，因此其酸性会比醇类物质高。

A carboxylic acid /ˌkɑːrbɒkˈsɪlɪk/ is an organic compound that contains a carboxyl group (C(O)OH). The general formula of a carboxylic acid is R–C(O)OH, with R referring to the rest of the (possibly quite large) molecule. Carboxylic acids occur widely and include the amino acids (which make up proteins) and acetic acid (which is part of vinegar and occurs in metabolism).