In this letter, the composite scattering of a plasma-coated target above a randomly rough sea surface is investigated by the finite-difference time-domain (FDTD) method with pulsed wave excitation. Both sea water and plasma layer are regarded as isotropic and dispersive media, and they satisfy the Debye model and the Drude model, respectively. The auxiliary differential equation (ADE) technique is incorporated into the FDTD scheme to deal with the dispersive media. To ensure the feasibility of our present method, the scattering of a perfect electrically conducting target without coating above dispersive sea surface by the ADE-FDTD method is compared with the result obtained by the parallel method of moments, which requires an individual run for every frequency of interest. Finally, the normalized radar cross section of a plasma-coated target above rough sea surface is calculated and analyzed for different parameters in detail.