In Haloferax volcanii, a series of Agl proteins mediates protein N-glycosylation. The genes encoding all but one of the Agl proteins are sequestered into a single gene island. The same region of the genome includes sequences also suspected but not yet verified as serving N-glycosylation roles, such as HVO_1526. In the following, HVO_1526, renamed AglS, is shown to be necessary for the addition of the final mannose subunit of the pentasaccharide N-linked to the surface (S)-layer glycoprotein, a convenient reporter of N-glycosylation in Hfx. volcanii. Relying on bioinformatics, topological analysis, gene deletion, mass spectrometry, and biochemical assays, AglS was shown to act as a dolichol phosphate-mannose mannosyltransferase, mediating the transfer of mannose from dolichol phosphate to the tetrasaccharide corresponding to the first four subunits of the pentasaccharide N-linked to the S-layer glycoprotein.