Reaction of indazoles with aldehydes in the presence of p-toluenesulfinic acid affords the corresponding sulfonyl indazoles in satisfactory yields. The reported Friedel-Crafts process is rather unusual on indazoles because of the reduced electronic density of the heterocycle. The obtained sulfonyl indazoles can be desulfonylated under reductive conditions, finally leading to 3-alkylated indazoles.