In enzymology, an icosanoyl-CoA synthase () is an enzyme that catalyzes the chemical reaction
The 5 substrates of this enzyme are stearoyl-CoA, malonyl-CoA, NADH, NADPH, and H<sup>+</sup>, whereas its 6 products are icosanoyl-CoA, CO<sub>2</sub>, CoA, NAD<sup>+</sup>, NADP<sup>+</sup>, and H<sub>2</sub>O.
This enzyme belongs to the family of transferases, specifically those acyltransferases transferring groups other than aminoacyl groups. The systematic name of this enzyme class is stearoyl-CoA:malonyl-CoA C-acyltransferase (decarboxylating, oxoacyl- and enoyl-reducing). Other names in common use include acyl-CoA elongase, C18-CoA elongase, and stearoyl-CoA elongase.