In this work, core–shell composites with Ni cores and ZnS nanowall shells have been successfully prepared by a hydrothermal deposition method. The crystal structures, microstructures and electromagnetic (EM) properties of the samples were investigated by X-ray diffraction, scanning electron microscopy, energy disperse X-ray spectrometry, transmission electron microscopy and a network analyser (2–18 GHz). In comparison with Ni and ZnS particles, the Ni/ZnS composites show the best microwave absorption properties. The optimal reflection loss is −25.78 dB at 14.24 GHz with the thickness of 2.7 mm. The reflection loss below −10 dB (90% microwave absorption) is 4.72 GHz (11.52–16.24 GHz) with only a thickness of 2.7 mm. The enhanced microwave absorption capabilities of Ni/ZnS composites were due to good electromagnetic impedance matching, interfacial relaxation and unique wall-like ZnS shells.