The weather in Thailand is beautiful. Ko Samui weather is always warm. Blue-sky days and a balmy ocean breeze are normal.  For the purists, late December to the end of March is the ideal tropical island weather experience. If you like it hot: April to June is the time Samui really bakes.  July to October is the ideal time to visit since the temperatures are quite mild. If you like the rainy season (not cold), try mid-October to early December.

Rainy Season
Koh Samui is a tropical island.  Inland parts of the island are a dense tropical forest.  Expect some rain at anytime in any month on Koh Samui. The time of heavier and more consistent rain is October through middle December.

Dry Season
When the rains stop and the plants grow wild and beautiful, this is called the Dry Season.  In Thailand, they call it Winter.  During this time, the temperatures are normally around 30C / 85F and sunny. This season is typically middle December through March.

Hot Season
When the ground has dried from a lack of rain and sand dusts the road, the Hot Season has started. The over-head tropical sun is brighter and hotter than usual.  Many tropical fruits ripen during this time.   This season starts in April and lasts until the cooling rains, which begin in September or October.