α-MnO₂ hollow-sphere-like clusters composed of nanotubes have been synthesized on a large scale by a simple hydrothermal method. The results show that the 3D hollow-sphere-like clusters are assembled by 1D nanotubes (HSC-nanotubes) with diameter of around 80–100 nm and length of around 2–3 μm. The influences of reaction time and temperature on the morphology of the final products are investigated, while the formation mechanism is also discussed. The α-MnO₂ hollow-sphere-like clusters possess much better dielectric properties than the bulk material, which is directly related to their special nanostructures. Besides, the synthesized α-MnO₂ hollow-sphere-like clusters were used as fillers in the fabrication of α-MnO₂/PVDF nanocomposites, which have excellent wave-absorption properties with low filler loading. The enhanced properties are caused by a synergic effect between α-MnO₂ and PVDF, as explained in this research.