Chemoselective N-benzenesulfonylation of aliphatic amines using 2-(4-nitrobenzenesulfonyl)-4,5-dichloropyridazin-3(2H)-one (2) gave the corresponding 4-nitrobenzenesulfonamides in good or excellent yield. This method is a simple, mild and general procedure for the chemoselective N-benzenesulfonylation of aliphatic amines.