A condensation reaction is a type of chemical reaction in which two simple molecules combine to form a third more complex molecule, with the elimination of a simple substance. The molecules of the substance produced are larger than those of the reactant molecules, while the eliminated substance (typically water or ammonia) consists of very small molecules.

Condensation reactions occur mainly in organic chemistry. An example is the reaction between a ketone and hydroxylamine to produce an oxime and water. A similar process involving monomers (polymer building blocks) can be used to produce a polymer in a type of reaction called condensation polymerization. This is the process by which subunits, such as amino acids and monosaccharides, join together to form biochemical polymers. See also addition reaction.