Cool Season

December - February is the cool season on the island. This is generally the best Time to visit for those who do not appreciate hot temperatures as it rarely exceeds 32 degrees. Weather varies from cloudy, windy to perfect sunny days. You may still encounter rainy days at the beginning of December but it will not affect your holidays and you will still be able to visit the island and enjoy the beach.s

Hot Season

March - August: The weather is dry during this period,and it is usually very sunny and hot. Temperatures can go up to 38 degrees during the hottest days of April.In May it might rain a little more though. This is a great period to visit the islands as you will be able to see Samui as its best. Do not miss Thailand's new year, Songkran, mid-April where you get free showers all day long!

Rain Season

September - November: Also known as the monsoon season although it is far to be as bad as in the rest of the country. It's still hot and the weather generally clears up with extended periods of dry and often sunny weather. Frequent showers, specially in the late afternoon and early morning will cool off your days.During this period, sea water will not be as hot as during the rest of the year but it will not go under 26 degrees.

Climate data for Ko Samui, Thailand
Month
Average high °C (°F)
Average low °C (°F)
Precipitation mm (inches)
Jan
29.0
(84.2)
24.1
(75.4)
137.8
(5.425)
Feb
29.5
(85.1)
25.0
(77.0)
57.8
(2.276)
Mar
30.7
(87.3)
25.6
(78.1)
77.8
(3.063)
Apr
32.1
(89.8)
26.1
(79.0)
76.6
(3.016)
May
32.6
(90.7)
25.8
(78.4)
46.5
(5.768)
Jun
32.2
(90.0)
25.5
(77.9)
112.7
(4.437)
Jul
32.0
(89.6)
25.1
(77.2)
122.8
(4.835)
Aug
31.9
(89.4)
25.1
(77.2)
118.7
(4.673)
Sep
31.6
(88.9)
24.8
(76.6)
116.8
(4.598)
Oct
30.5
(86.9)
24.4
(75.9)
290.2
(11.425)
Nov
29.5
(85.1)
24.1
(75.4)
489.6
(19.276)
Dec
24.1
(75.4)
23.9
(75.0)
209.1
(8.232)
Year
32.6
(90.7)
23.9
(75.0)
1,956.4
(77.024)
Source: World Weather Information Service: Ko Samui, Thailand