A simple and efficient method for the sulfonation of nitrobenzene using SO₃ as the sulfonating agent in a microreactor was developed and conducted in this work. The reaction enthalpy was determined using a microreactor-based calorimeter. The effects of molar ratio of reactants, reaction temperature and liquid hourly space velocity (LHSV) on the reaction performance were experimentally studied under solvent-free conditions. Under optimized reaction conditions, 94% conversion of nitrobenzene (NB) and 88% yield of meta-nitrobenzenesulfonic acid (m-NBSA) was obtained even when the residence time was less than 2 s. A microreactor-batch set-up was fabricated and developed to completely convert NB. The process safety has been improved and reaction time has been reduced compared to the batch method. These results demonstrate that the multifunctional integrated microreactor is an extremely useful and highly efficient tool for the sulfonation of NB under solvent-free conditions.