N-Glycosylation is an important post-translational modification in most secreted and membrane-bound proteins in eukaryotic cells. However, the insect N-glycosylation pathway and the potentials contributing to the N-glycan synthesis are still unclear because most of the studies on these subjects have focused on mammals and plants. Here, we identified Bombyx mori sialyltransferase (BmST), which is a Golgi-localized glycosyltransferase and which can modify N-glycans. BmST was ubiquitously expressed in different organs and in various stages of development and localized at the Golgi. Biochemical analysis using Sf9-expressed BmST revealed that BmST encoded α2,6-sialyltransferase and transferred N-acetylneuraminic acid (NeuAc) to the nonreducing terminus of Galβ1-R, but exhibited the highest activity toward GalNAcβ1,4-GlcNAc-R. Unlike human α2,6-sialyltransferase, BmST required the post-translational modification, especially N-glycosylation, for its full activity. N-Glycoprotein analysis of B. mori fifth instar larvae revealed that high-mannose-type structure was predominant and GlcNAc-linked and fucosylated structures were observed but endogenous galactosyl-, N-acetylgalactosaminyl- and sialyl-N-glycoproteins were undetectable under the standard analytical approach. These results indicate that B. mori genome encodes an α2,6-sialyltransferase, but further investigations of the sialylation potentials are necessary.