High-entropy diboride has been arousing considerable interest in recent years. However, the low toughness and damage tolerance limit its applications as ultra-high-temperature structural materials. Here we report that a unique SiB₆ additive has been first incorporated as boron and silicon sources to fabricate a high-entropy boride (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂–SiC composite though one-step boro/carbothermal reduction reactive sintering. A synergetic effect of high-entropy sluggish diffusion and SiC secondary phase retarded the grain growth of the (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂–51SiC composites. The small grain size was beneficial to shorten the diffusion path for mass transport, thereby enhancing the relative density to ~99.3%. These results in an increase of fracture toughness from ~5.2 in HEBS-1900 to ~7.7 MPa·m^1/2 in HEBS-2000, which corresponded to a large improvement of 48%. The improvement was attributed to a mixed mode of intergranular and transgranular cracking for offering effective pinning in crack propagation, resulting from balanced grain boundary strength collectively affected by improved densification, solid solution strengthening, and incorporation of SiC secondary phase.