Abstract: W-type barium–nickel ferrite (BaNi₂Fe₁₆O₂₇) is a highly promising material for electromagnetic wave (EMW) absorption because of its magnetic loss capability for EMW, low cost, large-scale production potential, high-temperature resistance, and excellent chemical stability. However, the poor dielectric loss of magnetic ferrites hampers their utilization, hindering enhancement in their EMW-absorption performance. Developing efficient strategies that improve the EMW-absorption performance of ferrite is highly desired but remains challenging. Here, an efficient strategy substituting Ba²⁺ with rare earth La³⁺ in W-type ferrite was proposed for the preparation of novel La-substituted ferrites (Ba_1−xLa_xNi₂Fe_15.4O₂₇). The influences of La³⁺ substitution on ferrites’ EMW-absorption performance and the dissipative mechanism toward EMW were systematically explored and discussed. La³⁺ efficiently induced lattice defects, enhanced defect-induced polarization, and slightly reduced the ferrites’ bandgap, enhancing the dielectric properties of the ferrites. La³⁺ also enhanced the ferromagnetic resonance loss and strengthened magnetic properties. These effects considerably improved the EMW-absorption performance of Ba_1−xLa_xNi₂Fe_15.4O₂₇ compared with pure W-type ferrites. When x = 0.2, the best EMW-absorption performance was achieved with a minimum reflection loss of −55.6 dB and effective absorption bandwidth (EAB) of 3.44 GHz.