Glycosyltransferases catalyze the reaction between an activated sugar donor and an acceptor to form a new glycosidic linkage. Glycosyltransferases are responsible for the assembly of oligosaccharides in vivo and are also important for the in vitro synthesis of these biomolecules. However, the functional identification and characterization of new glycosyltransferases is difficult and tedious. This paper describes an approach that combines arrays of reactions on an immobilized array of acceptors with an analysis by mass spectrometry to screen putative glycosyltransferases. A total of 14,280 combinations of a glycosyltransferase, an acceptor and a donor in four buffer conditions were screened, leading to the identification and characterization of four new glycosyltransferases. This work is notable because it provides a label-free method for the rapid functional annotation of putative enzymes.