To evaluate malodor reduction by means of the hydrothermal torrefaction, concentrations of the main malodorous compounds in odor samples from solid products before and after treatment were measured using gas chromatography. The concentrations of the sulfur-containing compounds were decreased by this treatment. To elucidate the mechanisms of this malodor reduction, odor distributions were analyzed with an odor-measuring device. The odor intensities in the odor sample from the solid product were lower than those from the dewatered sewage sludge (DSS), whereas the odor intensities in samples from liquid and gaseous products were as high as or higher than those from the DSS. Therefore, we conclude that one of the main malodor reduction mechanisms is reactive distillation: malodorous compounds are separated from the solid product by the inherent thermal effect of the hydrothermal torrefaction. In addition, the masking effect of odorous compounds produced as by-products in this treatment process and the sterilization of microbes in DSS probably play important roles in malodor reduction as well.