Frontolysis mechanisms by which surface heat flux relaxes the sea surface temperature (SST) front in the eastern Japan Sea (JS) are investigated in detail using observational datasets. On the warm southern side of the front, larger air–sea specific humidity and temperature differences induce stronger turbulent heat release compared to the cool northern side. As a result, stronger wintertime cooling and weaker summertime warming occur south of the front, and the meridional gradient in the surface net heat flux (NHF) tends to relax the SST front throughout the year. In the mixed-layer deepening phase (September–January), a higher entrainment velocity occurs on the warm southern side because of weaker stratification. Since the resulting thicker mixed layer on the southern side is less sensitive to surface cooling, the mixed layer depth (MLD) gradient damps the frontolysis by the NHF gradient. In the shoaling phase (April–June), a deeper mixed layer south of the front is caused by the weaker warming and reduced sensitivity of the thicker mixed layer to a shoaling effect by shortwave radiation. Owing to weaker sensitivity of the thicker mixed layer on the southern side to surface warming, the MLD gradient enhances the frontolysis by the NHF gradient. Therefore, it is shown that the mixed layer processes cause seasonality of weaker (stronger) frontolysis by surface heat fluxes, damping (enhancing) the frontolysis by the NHF gradient in winter (summer). This study reveals unique features of the frontolysis in the eastern JS compared with the Agulhas Return Current and Kuroshio Extension regions.