In this study, the waste heat recovery performance of a hexagonal thermoelectric generator (TEG) was investigated experimentally. Based on the system configuration and dimensions, a total of 18 thermoelectric modules (TEMs) were used for thermal energy harvesting. The internal finned structures of the hexagonal TEG were placed on the inner surface of the exhaust gas channel, in consideration of the thermal resistance and friction factor, in order to augment the heat transfer from the exhaust gas to the thermoelectric modules (TEMs), under various operating conditions. The surface of the TEMs was positioned on the hexagonal-shaped exhaust gas channel surface, whereas the other surface of the TEMs was maintained cool by a set of coolers through which the engine coolant flowed. The coolant line was modified in order to induce a total coolant flow of 10 LPM at 353 K from a coolant controller to the hexagonal TEG. In this study, the assembled hexagonal TEG was installed in the middle of the exhaust gas pipe of the internal combustion engine manufactured for hybrid electric vehicles, and was used as the heat source. The waste heat recovery performance of the hexagonal TEG was evaluated by considering eight engine-operating conditions under which the automotive vehicles were most frequently driven. The output power of the hexagonal TEG tended to increase with the increase in engine speed and load, and ranged between 21.2 and 98.8 W, which corresponded to a conversion efficiency of similar to 1.3-2.6%. An acceptable maximum pressure drop across the hexagonal TEG of similar to 2.1 kPa was observed for the maximum exhaust gas flow rate condition.