Sulfonic acid is an organic compound of general formula RSO₃H, where R is a hydrocarbon group. Examples are ethane sulfonic acid, C₂H₅5.O₃H, and benzene-sulfonic acid, C₆H₅5.O₃H.

Sulfonic acids are strong, water-soluble acids often used as their sodium salts, sulfonates. Aromatic sulfonic acids are made by sulfonation with fuming sulfuric acid. They are used to make detergents, dyes, sulfa drugs, and ion-exchange resins. Sulfonic acids are useful in synthesis, since the sulfonate group is readily replaced.