With a resurgence of covalent drugs, there is an urgent need for the identification of new moieties capable of cysteine bond formation. Herein, we report on the <i>N</i>-acylamino saccharin moieties capable of novel covalent reactions with cysteine. Their utility as alternative electrophilic warheads was demonstrated through the covalent modification of fructose-1,6-bisphosphatase (FBPase), a promising target associated with cancer and type 2 diabetes. The cocrystal structure of title compound <b>W8</b> bound with FBPase unexpectedly revealed that the <i>N</i>-acylamino saccharin moiety worked as an electrophile warhead that covalently modified the noncatalytic C128 site in FBPase while releasing saccharin, suggesting a previously undiscovered covalent reaction mechanism of saccharin derivatives with cysteine. Treatment of title compound <b>W8</b> displayed potent inhibition of glucose production in vitro and in vivo. This newly discovered reactive warhead supplements the current repertoire of cysteine covalent modifiers while avoiding some of the limitations generally associated with established moieties.