In enzymology, a starch synthase (EC 2.4.1.21) is an enzyme that catalyzes the chemical reaction Thus, the two substrates of this enzyme are ADP-glucose and a chain of D-glucose residues joined by 1,4-alpha-glycosidic bonds, whereas its two products are ADP and an elongated chain of glucose residues.

This enzyme belongs to the family of hexosyltransferases, specifically the glycosyltransferases.

Other names in common use include ADP-glucose-starch glucosyltransferase, adenosine diphosphate glucose-starch glucosyltransferase, adenosine diphosphoglucose-starch glucosyltransferase, ADP-glucose starch synthase, ADP-glucose synthase, ADP-glucose transglucosylase, ADP-glucose-starch glucosyltransferase, ADPG starch synthetase, and ADPG-starch glucosyltransferase Five isoforms seems to be present.

New work implies that SS4 is important for granule initiation.

(Szydlowski et al., 2011) As of late 2007, 4 structures have been solved for this class of enzymes, with PDB accession codes 1RZU, 1RZV, 2BFW, and 2BIS.