January - February
is the cool season on Samui island, temperatures average a pleasant 79°F / 26°C. This is generally the best time to visit as it is not too hot.
March - May
is the hot season, the temperature starts warming up, it has been known to reach 40°C There is generally less rain on Koh Samui during these months.
June - September
generally speaking, between these months, most days will be filled with sun, except for the occasional downpour, which usually doesn't last long.
October - December
frequent showers (not monsoons) come in October, and last until sometime around mid December, when the Samui weather generally clears up with extended periods of dry and often sunny weather. Water sports enthusiasts should note that the sea can get rough at this time, and visibility can be affected.
Samui Climate and Seasons
Dry Season
December - February: The dry season on Samui. Not much raining, but it can be a some refreshing wind or breeze on these islands then. Great for body surfing. The sun is shining a lot on us at this period of the year!
Hot Season
March - August: The weather is still dry in this time, and rather hot. In May the raining may increase a little. This is a great period to visit Samui if you can stand 30-35 degrees Celsius daytime. The sunshine is still dominating the day time, almost as much as in the Dry Season.
Rain Season
September - November: Also known as the monsoon season. It's still hot and normally, there are many sunny days anyway. Sometimes you might find yourself in a refreshing monsoon shower for a moment. The rain showers often last just for a short while though and as these island's water supplies are finite, the water is needed.
Notice!
The climate seasons are not exactly the same as in Krabi and Phuket, as these destinations are located in/at the Andaman Sea and Koh Samui, Koh Phangan and Koh Tao are located in the Gulf of Thailand. This makes the climate differences evident.
