Family 1 glycosyltransferases (GT1s, UGTs) form natural product glycosides with exquisite control over regio- and stereoselectivity, representing attractive biotechnological targets. However, regioselectivity cannot be predicted and large-scale activity assessment efforts of UGTs are commonly performed via mass spectrometry or indirect assays that are blind to regioselectivity. Here, we present a large high performance liquid chromatography screening discriminating between regioisomeric products of 40 diverse UGTs (28.6% average pairwise sequence identity) against 32 polyphenols, identifying enzymes able to reach high glycosylation yields (≥90% in 24 h) in 26/32 cases. In reactions with >50% yield, we observed perfect regioselectivity for 47% (75/158) on polyphenols presenting two hydroxyl groups and for 30% (43/143) on polyphenols presenting ≥3 hydroxyl groups. Moreover, we developed a nuclear magnetic resonance-based procedure to identify the site of glycosylation directly on enzymatic mixtures. We further selected seven regiospecific reactions catalyzed by four enzymes on five dihydroxycoumarins. We characterized the four enzymes, showing that temperature optima are functions of the acceptor substrate, varying by up to 20 °C for the same enzyme. Furthermore, we performed short molecular dynamics simulations of 311 ternary complexes (UGT, UDP-Glc, and glycosyl acceptor) to investigate the molecular basis for regioselectivity. Interestingly, it appeared that most UGTs can accommodate acceptors in configurations favorable to the glycosylation of either hydroxyl. In contrast, evaluation of hydroxyl nucleophilicity appeared to be a strong predictor of the hydroxyl predominantly glycosylated by most enzymes.