In enzymology, an alginate synthase (EC 2.4.1.33) is an enzyme that catalyzes the chemical reaction

GDP-D-mannuronate + (alginate)n ${\displaystyle \rightleftharpoons }$ GDP + (alginate)n⁺1

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.