In chemistry, a monomer (from Greek mono "one" and meros "part") is any of several small molecular structures that may be chemically bonded together to form long multi-part polymer molecules.

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 condensation reaction because of the loss of a hydrogen atom and a hydroxyl (-OH) group from the two monomer units. The bond between the monomers is an oxygen molecule with a bond with each monomer unit. The result of this reaction is H2O, also known as water.

Also note that multiple units built up from monomers are called polymers, but can also be called dimers, trimers, tetramers, pentamers, octamers if they have 2, 3, 4, 5, or 8 monomer units. The appropriate prefix prepended to 'mers' can refer to any number of monomer units, eg, decamer, being a 10-unit monomer chain or polymer.