Abstract: Interface modulation is an important pathway for highly efficient electromagnetic wave absorption. Herein, tailored interfaces between Fe₃O₄ particles and the hexagonal-YFeO₃ (h-YFeO₃) framework were constructed via facile self-assembly. The resulting interfacial electron rearrangement at the heterojunction led to enhanced dielectric and magnetic loss synergy. Experimental results and density function theory (DFT) simulations demonstrate a transition in electrical properties from a half-metallic monophase to metallic Fe₃O₄/h-YFeO₃ composites, emphasizing the advantages of the formed heterointerface. The transformation of electron behavior is also accompanied by a redistribution of electrons at the Fe₃O₄–h-YFeO₃ heterojunction, leading to the accumulation of localized electrons around the Y–O–Fe band bridge, consequently enhancing the polarization. A minimum reflection loss of −34.0 dB can be achieved at 12.0 GHz and 2.0 mm thickness with an effective bandwidth of 3.3 GHz due to the abundant interfaces, enhanced polarization, and rational impedance. Thus, the synergistic effects endow the Fe₃O₄/h-YFeO₃ composites with high performance and tunable functional properties for efficient electromagnetic absorption.