Exo-β-D-glucosaminidase (EC 3.2.1.165) from Photobacterium profundum (PpGlcNase) is an inverting GH (glycoside hydrolase) belonging to family 9. We have determined the three-dimensional structure of PpGlcNase to describe the first structure–function relationship of an exo-type GH9 glycosidase. PpGlcNase has a narrow and straight active-site pocket, in contrast with the long glycan-binding cleft of a GH9 endoglucanase. This is because PpGlcNase has a long loop, which blocks the position corresponding to subsites −4 to −2 of the endoglucanase. The pocket shape of PpGlcNase explains its substrate preference for a β1,4-linkage at the non-reducing terminus. Asp139, Asp143 and Glu555 in the active site were located near the β-O1 hydroxy group of GlcN (D-glucosamine), with Asp139 and Asp143 holding a nucleophilic water molecule for hydrolysis. The D139A, D143A and E555A mutants significantly decreased hydrolytic activity, indicating their essential role. Of these mutants, D139A exclusively exhibited glycosynthase activity using α-GlcN-F (α-D-glucosaminyl fluoride) and GlcN as substrates, to produce (GlcN)2. Using saturation mutagenesis at Asp139, we obtained D139E as the best glycosynthase. Compared with the wild-type, the hydrolytic activity of D139E was significantly suppressed (<0.1%), and the F−-release activity also decreased (<3%). Therefore the glycosynthase activity of D139E was lower than that of glycosynthases created previously from other inverting GHs. Mutation at the nucleophilic water holder is a general strategy for creating an effective glycosynthase from inverting GHs. However, for GH9, where two acidic residues seem to share the catalytic base role, mutation of Asp139 might inevitably reduce F−-release activity.