Mammalian sialyltransferases transfer sialic acids onto glycoproteins and glycolipids within the Golgi apparatus. Despite their key role in glycosylation, the study of their enzymatic activities is limited by the lack of appropriate tools. Herein, we developed a quick and sensitive sialyltransferase microplate assay based on the use of the unnatural CMP-SiaNAl donor substrate. In this assay, an appropriate acceptor glycoprotein is coated on the bottom of 96-well plate and the sialyltransferase activity is assessed using CMP-SiaNAl. The alkyne tag of SiaNAl enables subsequent covalent ligation of an azido-biotin probe via CuAAC and an antibiotin-HRP conjugated antibody is then used to quantify the amount of transferred SiaNAl by a colorimetric titration. With this test, we evaluated the kinetic characteristics and substrate preferences of two human sialyltransferases, ST6Gal I and ST3Gal I toward a panel of asialoglycoprotein acceptors, and identified cations that display a sialyltransferase inhibitory effect.