Thermal properties were characterized for zirconium diboride produced by reactive hot pressing and compared to ZrB2 ceramics that were hot pressed from commercial powders. No sintering additives were used in either process. Thermal conductivity was calculated from measured values of heat capacity, thermal diffusivity, and density for temperatures ranging from 298 to 2273 K. ZrB2 produced by reactive hot pressing achieved near full density, but had a small volume fraction of ZrO2, whereas hot-pressed ZrB2 contained porosity and carbon inclusions. Reactive hot pressing produced a ceramic with higher thermal diffusivity and heat capacity, resulting in thermal conductivities of 127 W·(m·K)-1 at 298 K and 80 W·(m·K)-1 at 2273 K, which were up to ~30% higher than typically reported for hot-pressed ZrB2.