Most of Shimoshima Island is administered as part of Amakusa city, with 67 km2 patch on north-west coast belonging to the town of Reihoku.

Shimoshima Island is located in the humid subtropical climate zone (Köppen Cfa), with four distinct seasons.

During the winter, the higher elevations see some snowfall, but the lower, populated areas seldom do.

Spring in Shimoshima Island starts off mild, but ends up being hot and humid.

The island's weather is affected by the nearby Liman current [ja] while being shielded from the warm Kuroshio Current by a part of the Kyushu island, resulting in wetter and colder climate than should be expected at lower 30's latitudes.