The effect of chromium admixture on the processes in the HfB2-SiC ceramic powder system during its pressureless sintering at 1600 °C was studied. It was shown that an increase in chromium content from 0% to 15.5% in the HfB2-SiC ceramic powder mixture leads to a continuous increase in its relative density up to 90%. A transient liquid phase Cr-Si-C-B is formed at 1600 °C, and it promotes intense sintering of HfB2 and SiC powders. The oxidation resistance of HfB2-SiC-Cr ceramics was studied in static air at 1000–1500 °C. It was shown that the oxidation resistance is greatly improved due to a decrease in the porosity of the sintered ceramic system because of chromium additive. The presence of chromium oxide in the formed surface glassy layer can also lead to the increase in the oxidation resistance. These results suggest that chromium can be considered as a promising sintering additive for HfB2-SiC and similar systems.