Condensation reaction, in chemistry, a reaction in which the reactants are combined into a single product with the loss of a small molecule such as water or occasionally another stable compound. Condensation reactions are typically catalyzed, or accelerated, by heat, radiation, or acids.

Condensation reactions are extremely common in organic (carbon-containing) chemistry, in which they are particularly important in the synthesis of polymers and other complex compounds. For example, the amino acids that make up proteins are joined together by peptide bonds formed through condensation reactions. Likewise, the synthesis of carbohydrates involves the formation of glycosidic bonds by condensation reactions between monosaccharides. The formation of lipids and waxes from fatty acids and alcohols also occurs through condensation reactions.

Condensation reactions are also commonly used to convert compounds that contain hydroxyl (−OH) groups into compounds with carbonyl groups (C=O). For example, the reaction of an alcohol with a carboxylic acid gives an ester and water (an alcohol also condenses with a carboxylic acid to give an ester and water). In this case, the hydroxyl group of the alcohol and the carboxyl group of the acid react to form water, while the remaining atoms from the two compounds are joined together to form the ester.