AbstractSerine proteases are one of the largest groups of enzymes, found in both eukaryotes and prokaryotes, and are responsible for many different functions. The detailed information about the hydrogen-bonds in the catalytic triad (Asp…His…Ser) of these enzymes is of importance in order to fully understand the mechanism of action. The aspartate of the triad is hydrogen bonded to the histidine but the exact nature of this bond has been under discussion for some time. It is either a common short ionic hydrogen bond (SIHB) or a delocalized low barrier hydrogen bond (LBHB) were the hydrogen bond is shorter. So far, the evidence for LBHB in proteins have not been conclusive. Here we show clear NMR evidence that LBHB does exist in NS3, a serine protease from Dengue. The one bond coupling constant between the hydrogen and nitrogen was shown to be only 52 Hz instead of the usual 90 Hz. This together with a 1H chemical shift of 19.93 ppm is evidence that the hydrogen bond distance between His and Asp is shorter than for SIHB. Our result clearly shows the existence of LBHB and will help in understanding the mechanism of the catalytic triad in the important group of serine proteases.