In this work, the microwave absorption (MWA) performance of a Fe₃O₄–3D reduced porous carbon nanocomposite (3D rPC NC) in the X-band region is reported. Three different shields are fabricated by altering the ratio of Fe₃O₄ nanoparticles (NPs) and 3D rPC and evaluating their microwave (MW) shielding performance with appropriate in-wearing instruments due to their minimum thickness. The chemical interaction between Fe₃O₄ NPs and 3D rPC is examined from chemical composition analysis of Fe₃O₄–3D rPC (1 : 2 ratio), which is confirmed by the presence of the Fe–O–C bond in the O 1s spectrum obtained from XPS analysis and subsequent analysis using FESEM images. Furthermore, it is found from N₂ adsorption/desorption analysis that 3D rPC possesses a huge surface area of 787.312 m² g⁻¹ and showcases a type-V isotherm (mesoporous and/or microporous) behavior. The dielectric and magnetic losses of Fe₃O₄–3D rPC with a 1 : 2 ratio (tan δ_εr = 1.27 and tan δ_μr = 5.03) are higher than those of Fe₃O₄ NPs, 3D rPC and their NCs due to its magnetic and electrical conducting pathways modifying the material's polarization and dipole moment. The lightweight, polymer-free Fe₃O₄–3D rPC (1 : 2) NCs with minimum thickness on the order of 0.5 mm exhibited a higher total shielding effectiveness (SE_T = 41.285 dB), and it effectively blocked 99.9963% of the transmittance due to electric and magnetic polarization resulting from the presence of a heterogeneous interface surface.