Koh Samui's climate is tropical. It is high in both temperature and humidity. The average temperature is about 30 degrees Celsius or 86 degrees Fahrenheit. Even though it is more or less summer all year round, you could say that Thailand and Koh Samui has three seasons:
December - February is called the the dry season on Koh Samui. Normally not much rain, but it can be some refreshing winds or breeze on the island at this time. Great for sailing and other water sport activities. Also, the sun is shining a lot at this time of year and the water conditions are usually great for swimming.
March - August is the hot season in Koh Samui. The weather is very dry and rather hot in this time of year. In May and June however, you can experience a little increase in the rain. This is a great period to visit Samui, if you can stand 30-35 degrees Celsius daytime, as the Thai New Years Celebration, Songkran, is celebrated between the 12th and 14th of April. This is the largest water splashing festival in the whole of South East Asia.
September - November is also known as the monsoon season. It is still hot in the daytime and, normally, there are many sunny days. However, you can experience that the evenings are getting cooler. Sometimes the refreshing monsoon showers only last for a moment and then it clears up again. So, every year the monsoon is different. From a whole week of rain non stop to daily afternoon showers. No year is the same.
All you need to know. Temperatures in Celsius. Average Values