Ultrahigh-temperature hafnium diboride ceramics with additions of 15 vol.% MoSi₂ or 15 vol.% SiC or a combined addition of 15 vol.% SiC and 5 vol.% WC were produced by hot pressing in the range 1800–2000°C. The density of the produced composite ceramics was >98%. The components interacted in the hot pressing process to form new high-temperature phases (WB, MoB). The graine size of all structural elements did not exceed 5 μm. The maximum bending strength was reached by the HfB₂–15 vol.% SiC–5 vol.% WC samples: 587 ± 25 MPa at room temperature and 535 ± 18 MPa at a test temperature of 1800°C, being associated with transcrystalline fracture of the ceramics. A three-layer oxide film formed: the upper layer was borosilicate glass with a HfSiO₄ interlayer, the middle layer was HfO₂ with B₂O₃–SiO₂ inclusions, and the lower layer consisted of hafnium oxide and inclusions of other oxides. The total thickness of the oxide film was ~50 μm for the material oxidized at 1600°C for 5 h and ~150 μm at 1500°C for 50 h. The highest oxidation resistance was acquired by the HfB₂–15 vol.% MoSi₂ composite, where the oxidation rate did not exceed ~1 mg/cm² ∙ h because a dense and homogeneous HfSiO₄ layer developed on the surface. However, the most corrosion-resistant zirconium diboride composite, ZrB₂–15 vol.% MoSi₂, showed an oxidation rate of ~2 mg/cm² ∙ h. This high oxidation resistance of the hafnium diboride ceramics is explained by slower oxygen diffusion in HfO₂ and HfSiO₄ than in ZrO₂ and ZrSiO₄, which is confirmed by mathematical modeling of the oxidation process.