In chemistry, a trimer (, "three" + , "parts") is an oligomer derived from three identical precursors. An example is the procedure of production of polymers. At first, a monomer is made. By combining two monomers, a dimer is produced. With further additions, a trimer and eventually a polymer is made. Often, trimerization competes with polymerization; for example, dimethylsilanediol polymerizes to polydimethylsiloxane, even though a trimer is made:

Me₂Si(OH)₂ + (HO)₂SiMe₂ → Me₂(OH)Si-O-Si(OH)Me₂ – dimer

Me₂(OH)Si-O-Si(OH)Me₂ + (HO)₂SiMe₂ → Me₂(OH)Si-O-SiMe₂-O-Si(OH)Me₂ – trimer

Me₂(OH)Si-O-SiMe₂-O-Si(OH)Me₂ + (HO)₂SiMe₂ → Me₂(OH)Si-O-SiMe₂-O-SiMe₂-O-Si(OH)Me₂ – tetramer

et cetera, until Me₂(OH)Si-O-[SiMe₂-O-]_nSi(OH)Me₂ (n>100) – polymer