The atmosphere–ocean-coupled regional downscaling system of the Regional Spectral Model for the atmosphere and the Regional Ocean Modeling System (RSM–ROMS) was used to improve the downscaling simulation accuracy, particularly of coastal areas, and a dynamical downscale of the historical global reanalysis data for the East Asian region over 25 years was conducted. The results showed that in the coupled run, the sea surface temperature (SST) tended to show large-scale discrepancy from reality, basically because the models remain imperfect. On the other hand, for net heat flux, precipitation, and surface air temperature, the coupled run showed positive improvement compared with the uncoupled run. The improvement in these three variables and the degradation in SST were also apparent for event-based (one-month) averages. This inconsistency between the impacts on SST and the other variables may indicate that there is room to improve the model system further, particularly in the coupling and/or boundary layer processes for both the atmosphere and ocean.