Hydrothermal electrolysis reactions of glycerol were investigated under various operating conditions to determine the effects of applied DC current, electrolysis time, and alkali concentration on the decomposition mechanism of glycerol. In addition, intermediate products were identified, possible reaction schemes for both hydrothermal electrolysis and hydrothermal degradation of glycerol based on experimental data were clarified, and detailed product analysis was conducted using high performance liquid chromatography (HPLC), gas chromatography with a flame ionization detector (GC-FID), and gas chromatography with a thermal conductivity detector (GC-TCD). For the present study, a continuous flow reactor equipped with titanium electrodes (as cathode and anode), an electric furnace, a heater, a pump, a heat exchanger, a back pressure regulator, and DC supply was used. The main gaseous product was hydrogen, whereas glycolaldehyde, lactic acid, and formic acid were the main liquid products. Results indicate that greater than 92% of the glycerol could be decomposed under optimum conditions by hydrothermal electrolysis using the continuous flow reactor.