SiOC-based ceramics are considered promising electromagnetic wave-absorbing materials because of their lightweight, high-temperature resistance, and heat insulation properties. Herein, SiOC@C ceramic nanospheres were prepared using a liquid-phase method combined with a polymer-derived ceramic (PDC) method, followed by heat treatment in N₂ and Ar atmospheres at different temperatures. The morphology, microstructure, phase composition, and electromagnetic wave absorption performance of the SiOC@C ceramic nanospheres were investigated in detail. The SiOC@C ceramic nanospheres obtained in the Ar atmosphere showed a minimum reflection loss (RL_min) of −67.03 dB, whereas the SiOC@C ceramic nanospheres obtained in the N₂ atmosphere exhibited an RL_min value of −63.76 dB. The outstanding electromagnetic wave absorption performance of the SiOC@C ceramic nanospheres was attributed to the synergistic effect between conductive loss, interfacial/defect polarization loss, multiple reflections, and scattering. Therefore, this research provides valuable insights into the design and fabrication of SiOC ceramic-based electromagnetic wave absorbers.