Koh Samui is a tropical island. Inland parts of Koh Samui are a dense tropical forest, so expect rain at anytime in any month on Koh Samui. The time of heavier and more consistent rain known as monsoon season is October through to mid December.
When the rains stop and the plants grow wild and beautiful is the dry season, or as the Thais calls it, winter. Winter on Koh Samui is usually 30C / 85F and sunny. This season is typically mid December through March.
When the ground has dried from a lack of rain and sand dusts the road, the hot season has started. The sun overhead is brighter and hotter than usual. Many tropical fruits ripen during this season. This season starts in April and lasts until the cooling rain, which begins in September / October.
This is the weather forecast for Koh Samui over the next few days: