To overcome the poor sinterability and low fracture toughness (K_IC) of zirconium carbide (ZrC) ceramics, a novel multiscale microstructure design was proposed via a two-step in situ reactive spark plasma sintering (SPS) process using ZrC, TiSi₂, and B₄C powders. During sintering, TiSi₂ preferentially reacted with B₄C to form TiB₂ and primary silicon carbide (SiC), while the released Si further reacted with ZrC to yield ZrSi₂ and secondary SiC. The two-step SPS (1600 °C/3 min + 1800 °C/10 min) promoted complete in situ reactions, liquid-phase sintering, and interdiffusion of Zr/Ti, leading to the formation of (Zr,Ti)C and (Ti,Zr)B₂ solid solutions. With the addition of 30 mol% TiSi₂ and 15 mol% B₄C, the multiphase ceramics exhibited a refined submicrostructure (grain size < 500 nm), achieving a high flexural strength of 824±46 MPa and K_IC of 7.5±0.5 MPa·m^1/2. The synergistic enhancement in strength and toughness is attributed to a multiscale strengthening/toughening mechanism: solid-solution strengthening at the atomic scale, effective grain boundary pinning by nanosized primary and secondary SiC particles at the nanoscale, and toughening through crack deflection and bridging by TiB₂–SiC agglomerates and the higher-toughness ZrSi₂ phase at the microscale. This work provides a viable and innovative approach for designing high-performance ultrahigh-temperature ceramics through tailored in situ reactions and microstructural control.