Yakushima receives extraordinary rainfall — the interior mountains catch moisture from both the Pacific and East China Sea, and the local saying that "it rains 35 days a month" is only a modest exaggeration. Rain is possible at any time of year, but the wettest period is May to September, when Pacific fronts bring extended downpours. Paradoxically, this is also the time when the forests are at their most lush and the waterfalls most dramatic.
The best windows for hiking are May–early June and September–October. May and early June see the rhododendrons blooming on the high ridges, and while rain is frequent, the air is cool enough for comfortable trekking. September and October are generally drier, with stable autumn weather. July and August bring summer crowds alongside heat and the risk of typhoons.
Winter (December–February) is cold at elevation and snowfall is possible on the high peaks, but the coast remains mild. The cedar forests are beautiful year-round. For sea turtle watching, come between May and October — nesting peaks in June and July, hatching from late July through September.