Boron carbide/hafnium diboride composites were prepared by spark plasma sintering of a mixture of hafnium diboride and boron carbide powders. Boron carbide was prepared with a 13.3 at.% composition of carbon, known as the ideal carbon content to maximize the dimensionless figure of merit. The hafnium diboride content was varied between 0 and 20% by weight, and the effect on the thermoelectric properties was studied. Addition of HfB2 generally yielded an increase in the electrical conductivity and simultaneously a reduction in thermal conductivity, indicating it has potential as an enhancer of thermoelectric properties. However, the increase in electrical conductivity was not as large as observed in some composite systems, since HfB2 turned out to be a poor sintering additive leading to lower relative densities, and was furthermore offset by a moderate decrease in Seebeck coefficient. For future composite design, the sintering characteristics of the additives can be concluded as an important additional parameter to be taken into account. The optimal hafnium diboride content for relatively dense samples was found to be 10 wt%, resulting in an improvement in the maximum figure of merit, up to ZT = 0.20 at 730 °C.