In enzymology, an alpha-1,3-glucan synthase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are UDP-glucose and [alpha-D-glucosyl-(1-3)]n, whereas its two products are UDP and [alpha-D-glucosyl-(1-3)]n+1.
This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is UDP-glucose:alpha-D-(1-3)-glucan 3-alpha-D-glucosyltransferase. Other names in common use include uridine diphosphoglucose-1,3-alpha-glucan glucosyltransferase, and 1,3-alpha-D-glucan synthase.