In enzymology, a phosphomannan mannosephosphotransferase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are GDP-mannose and (phosphomannan)n, whereas its two products are GMP and (phosphomannan)n+1.
This enzyme belongs to the family of transferases, specifically those transferring non-standard substituted phosphate groups. The systematic name of this enzyme class is GDP-mannose:phosphomannan mannose phosphotransferase.