Sea surface temperature (SST) is an important component of climate and weather systems at various time scales. Asym- metric seasonal SST variations in the Northwestern Pacific Marginal Seas (NWPMS) are investigated in this study using observation data and numerical model results. The asymmetry in SST seasonal variation is estimated quantitatively and com- pared with heat advection and surface net heat flux using SST data and atmospheric variables from the European Centre for Medium-Range Weather Forecast (ECMWF). The SST increases faster than it decreases, whereas air temperature increases slowly. Heat advection and surface heat flux were estimated using numerically modeled SST and ocean currents, which con- tribute to the asymmetry of seasonal SST variations. Heat advection shows good correlation with the SST seasonal variation asymmetry. Model results without currents along the boundary show more symmetrical SST variations. This suggests that heat advection is a prominent cause of asymmetry in the seasonal variation.