In enzymology, an undecaprenyl-phosphate mannosyltransferase (EC 2.4.1.54) is an enzyme that catalyzes the chemical reaction

GDP-mannose + undecaprenyl phosphate ${\displaystyle \rightleftharpoons }$ GDP + D-mannosyl-1-phosphoundecaprenol

Thus, the two substrates of this enzyme are GDP-mannose and undecaprenyl phosphate, whereas its two products are GDP and D-mannosyl-1-phosphoundecaprenol.

This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is GDP-mannose:undecaprenyl-phosphate D-mannosyltransferase. Other names in common use include guanosine diphosphomannose-undecaprenyl phosphate, mannosyltransferase, GDP mannose-undecaprenyl phosphate mannosyltransferase, and GDP-D-mannose:lipid phosphate transmannosylase. It employs one cofactor, phosphatidylglycerol. Sources of this enzyme includes Micrococcus luteus, Phaseolus aureus, Mycobacterium smegmatis and cotton fibers.

• Lahav M, Chiu TH, Lennarz WJ (1969). "Studies on the biosynthesis of mannan in Micrococcus lysodeikticus II. The enzymatic synthesis of mannosyl-l-phosphoryl-undecaprenol". J. Biol. Chem. 244 (21): 5890–8. PMID 5350943.