Nickel (Ni)-filled multiwall carbon nanotubes (MWCNTs) were synthesized by template-assisted electrochemical deposition of Ni in MWCNTs. Complex dielectric permittivity and magnetic permeability measurements of Ni nanowires (Ni NWs), MWCNTs, and Ni-filled MWCNTs were carried out in the S band. Ni-filled MWCNTs exhibited enhanced real (epsilon(1)) and imaginary (epsilon(2)) permittivity when compared to both Ni NWs and MWCNTs. This increase in epsilon(1) and epsilon(2) is attributed to the enhanced interfacial polarization, reduced eddy currents, and the presence of defects in MWCNTs. Ni-filled MWCNTs serve as an efficient microwave absorber in the S-band frequency regime.