In organic chemistry, the sulfonamide functional group (also spelled sulphonamide) is an organosulfur group with the structure . It consists of a sulfonyl group () connected to an amine group (). Relatively speaking this group is unreactive.  Because of the rigidity of the functional group, sulfonamides are typically crystalline; for this reason, the formation of a sulfonamide is a classic method to convert an amine into a crystalline derivative which can be identified by its melting point.  Many important drugs contain the sulfonamide group.
A sulfonamide (compound) is a chemical compound that contains this group. The general formula is  or , where each R is some organic group; for example, "methanesulfonamide" (where R = methane, R' = R" = hydrogen) is . Any sulfonamide can be considered as derived from a sulfonic acid by replacing a hydroxyl group () with an amine group.
In medicine, the term "sulfonamide" is sometimes used as a synonym for sulfa drug, a derivative or variation of sulfanilamide. The first sulfonamide was discovered in Germany in 1932.