Thermoelectricity is regarded as one of the promising waste heat recovery candidates. A fundamental but effective method for the best use of thermoelectric generators (TEGs) is to maximize the heat flow crossing thermoelectric materials. The main focus of the present study was to develop a thermal interface material (TIM) with high thermal conductivity and temperature resistance as the key to minimizing the overall thermal resistance of the heat flow path of a TEG module operating under high-temperature conditions. In combination with a polyimide matrix and a multi-dimensional filler compound, a new TIM having stable heat conduction behavior at high temperatures was produced. The developed TIM was stable without losing mass up to similar to 500 degrees C, and its thermal conductivity reached 81.4 W/m.K. It was applied to the interface between a TEG and a heat source whose temperature ranged from 100 to 300 degrees C and, the effect of the thermal conductivity and interface thermal resistance of the TIM on thermoelectric power generation performance during onsite curing of the TIM was investigated. Reduction of the interface thermal resistance by the new TIM improved the power generation by, at most, 132.3 and 38.6% compared to cases without a TIM and with a conventional graphite foil TIM, respectively. In terms of energy conversion efficiency, the TEG with the new TIM showed maximum improvements of 73.2 and 20.9% over the cases without a TIM and with a graphite TIM for the same temperature difference across the TEG, respectively. In addition, thermal cyclic testing confirmed the long-lasting heat-conducting feature of the developed TIM. The present work clearly shows the potentially significant influence that TIMs have on the waste heat recovery performance of TEGs.