Electrochemical desorption of a self-assembled monolayer (SAM) of n-alkanethiols was investigated in four kinds of ionic liquids. It was found for the first time by linear potential sweep voltammetry that reductive desorption of the SAM took place even in ionic liquids. The potential peak was negatively shifted with an increase in the alkyl chain length of the alkanethiol SAM in a manner similar to that for the case of reductive desorption of the SAM in an aqueous solution. However, the cathodic wave broadened with an increase in chain length, although its width became narrow in the case of measurements in an aqueous solution. The contrary behaviors of reductive desorption in an ionic liquid and in an aqueous solution were discussed by comparing the experimentally obtained voltammograms with numerically simulated ones. Based on the results obtained, we propose a plausible reaction scheme in which cations work mainly as charge compensators for generated alkanethiolates and anions work as a solvent.