Ultrasonic-assisted hot pressing (UAHP) has shown significant potential in enhancing both the densification and mechanical performance of metallic materials. However, the poor high-temperature stability of ultrasonic systems severely limits its application in the fabrication of high-melting-point materials. To fill this gap, UAHP was operated at temperatures exceeding 2000 °C and employed in the preparation of monolithic boron carbide (B₄C) ceramics for the first time. The densification behavior, microstructure evolution, and mechanical properties of B₄C fabricated via UAHP were systematically investigated and compared with those prepared by conventional hot pressing (HP). It was demonstrated that the introduction of high-frequency ultrasonic vibration in UAHP can not only accelerate the densification rate but also reduce the densification temperature and enhance the mechanical properties of B₄C. Specifically, the relative density of B₄C increased from 90.90% to 97.22% at 1900 °C under UAHP, which was comparable to that achieved by HP at 1950 °C, indicating a 50 °C reduction in densification temperature. In addition, a significant increase in densification efficiency by reducing the densification time during UAHP endowed B₄C with both near-full density and superior mechanical properties. The B₄C ceramics prepared by UAHP at 1950 °C for 20 min and at 2050 °C for 5 min exhibited flexural strengths of 669.3±19.4 and 688.3±32.5 MPa, respectively, and fracture toughnesses of 4.37±0.23 and 4.22±0.29 MPa·m^1/2, respectively. These results suggest that UAHP is a promising strategy for efficient densification and optimization of the mechanical properties of B₄C ceramic and opens a new avenue for the preparation of difficult sintering ceramics.