In this paper, the concept of incorporating core–shell structured units as secondary phases to toughen Al₂O₃ ceramics is proposed. Al₂O₃ composite ceramics toughened by B₄C@TiB₂ core–shell units are successfully synthesized using a combination of molten salt methodology and spark plasma sintering. The synthesis of B₄C@TiB₂ core–shell toughening units stems from the prior production of core–shell structural B₄C@TiB₂ powders, and this core–shell structure is effectively preserved within the Al₂O₃ matrix after sintering. The B₄C@TiB₂ core–shell toughening unit consists of a micron-sized B₄C core enclosed by a shell approximately 500 nm in thickness, composed of numerous nanosized TiB₂ grains. The regions surrounding these core–shell units exhibit distinct geometric structures and encompass multidimensional variations in phase composition, grain dimensions, and thermal expansion coefficients. Consequently, intricate stress distributions emerge, fostering the propagation of cracks in multiple dimensions. This behavior consumes a considerable amount of crack propagation energy, thereby enhancing the fracture toughness of the Al₂O₃ matrix. The resulting Al₂O₃ composite ceramics display relative density of 99.7%±0.2%, Vickers hardness of 21.5±0.8 GPa, and fracture toughness 6.92±0.22 MPa·m^1/2.