Glycoprotein endo-ñ-1,2-mannosidase (, glucosylmannosidase, endo-ñ-<small>D</small>-mannosidase, endo-ñ-mannosidase, endomannosidase, glucosyl mannosidase) is an enzyme with systematic name glycoprotein glucosylmannohydrolase. It catalyses the hydrolysis of the terminal ñ-<small>D</small>-glucosyl-(1,3)-<small>D</small>-mannosyl unit from the GlcMan<sub>9</sub>(GlcNAc)<sub>2</sub> oligosaccharide component of the glycoprotein produced in the Golgi membrane.
This protein is involved in the synthesis of glycoproteins.