March to May and October to November are the most comfortable months — warm but not yet oppressive, with relatively low rainfall. June to September are hot, humid, and prone to passing typhoons; ferries and flights can be cancelled at short notice.
The Iriomote sea is warm enough for snorkelling year-round, but visibility is best from April to October. December and January are cool by Japanese standards (16–20°C) and unusually pleasant for hiking.