In enzymology, an alginate synthase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are GDP-D-mannuronate and (alginate)n, whereas its two products are GDP and (alginate)n+1.
This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is GDP-D-mannuronate:alginate D-mannuronyltransferase. This enzyme is also called mannuronosyl transferase. This enzyme participates in fructose and mannose metabolism.