In chemistry, a monomer (from Greek mono "one" and meros "part") is a small molecule that may become chemically bonded to other monomers to form a polymer.

Examples of monomers are hydrocarbons such as the alkane, alkene, and alene (homologous) series. Other hydrocarbon monomers such as styrene and ethene form polymers to make plastics like polystyrene and polyethene.

Amino acids are natural monomers, and polymerize to form proteins. Glucose monomers can also polymerize to form starches, amilopectins and glycogen polymers. The polymerisation reaction is known as a dehydration or condensation reaction (due to the formation of water (H₂O) as one of the products) where a hydrogen atom and a hydroxyl (-OH) group are lost to form H₂O and an oxygen molecule bonds between each monomer unit.

Note that polymers built from monomers can also be called dimers, trimers, tetramers, pentamers, octamers.. if they have 2, 3, 4, 5, 8.. monomer units, respectively. Any number of these monomer units may be indicated by the appropriate prefix, eg, decamer, being a 10-unit monomer chain or polymer.