Core/shell structured Ni/Fe3O4 composites with grain sizes of 200–800 nm were synthesized by using a hydrothermal method. The phase structure, morphology, magnetic and electromagnetic absorption properties of the as-prepared samples were characterized by various analysis techniques. Experimental results revealed that spinel Fe3O4 was in situ deposition on the surface of Ni spheres through electrostatic interactions. Moreover, the Ni/Fe3O4 mass ratios had a significant influence on the electromagnetic absorption performances. When the mass ratio of Ni/Fe3O4 was 1:1, the composites reached an outstanding reflection loss (RL) of [Formula: see text]48.06[Formula: see text]dB at 12.96[Formula: see text]GHz with a thin thickness of 1.8[Formula: see text]mm. Meanwhile, the effective absorption bandwidth was 5.36[Formula: see text]GHz (10.72–16.08[Formula: see text]GHz), covering the X and Ku bands. The enhanced electromagnetic absorption performance of core/shell structured Ni/Fe3O4 composites as compared to pure Ni, may be attributed to the multiple interfacial polarization, magnetic exchange coupling resonance and optimal impedance matching. Consequently, core/shell structured Ni/Fe3O4 composites would be promising candidates for a wider range of electromagnetic absorption applications.