Although there is much evidence of land-sea ecological connectivity, a systematic quantitative understanding is still lacking. The ecohydrological function of forests ensures regulation of land-sea connectivity in watersheds, but there is currently no generalized evidence of the relationship between land-sea connectivity and forest dynamics. Here, we evaluated the connectivity between forest and coastal ecosystems in 160 watersheds located within Japan from latitudes 31 to 43 degrees north using satellite observation data from 2003 to 2018. The connectivity between forests and coastal ecosystems tended to follow the environmental gradients of forest ecosystems along the climatic zone. It was also shown that watersheds with spatiotemporally stable forest ecosystems have a high probability of connectivity.