In Ko Samui the climate is tropical, hot all year round, with a long rainy season, due to the monsoon of Southeast Asia, which runs from May to mid-January, and a drier season, from mid-January to April. The driest months are February and March, in which, however, some showers can sometimes occur.
Rainfall amounts to about 1,900 millimetres (77 inches) per year, and the brevity of the dry season allows the forest to grow in the interior of the island. Within the rainy season we can distinguish between two periods: at the beginning, in May, there's a first peak, then the rains are quite abundant but without excesses from June to September, when they occur in the form of showers or downpours, usually brief, and finally, between October and December the true monsoonal peak occurs, when the rains are torrential, especially in November. This happens because in the last months of the year the northeast monsoon, or retreating monsoon, starts to blow, affecting the coasts exposed to the wind coming from the sea, such as this island which is located a short distance from the Peninsular Thailand.
Here is the average precipitation.
Average precipitation - Ko Samui

Ko Samui	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Ko Samui	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	125	40	55	85	170	120	145	135	140	275	465	190	1945
Prec. (in)	4.9	1.6	2.2	3.3	6.7	4.7	5.7	5.3	5.5	10.8	18.3	7.5	76.6
Days	9	4	4	8	17	16	16	16	18	20	20	15	163

As mentioned, the temperature is high throughout the year, around 24/25 °C (75/77 °F) at night, and 30 °C (86 °F) during the day, however, the coolest months are December and January, while from February to May a rise in temperature is observed, though less marked than in the mainland of Thailand; April and May are, albeit slightly, the hottest months. In the summer months the heat continues to be intense and sultry, though tempered by sea breezes, as happens in the rest of the year.
Here are the average temperatures.
Average temperatures - Ko Samui

Ko Samui	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Ko Samui	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	24	25	26	26	26	26	25	25	25	24	24	24
Max (°C)	29	30	31	32	33	32	32	32	32	31	30	29
Min (°F)	75	77	79	79	79	79	77	77	77	75	75	75
Max (°F)	84	86	88	90	91	90	90	90	90	88	86	84

The amount of sunshine is good in the driest period, from mid-January to April, while in the rest of the year the sun often shines, but it alternates with some cloudiness, more or less widespread, and associated with rain showers.
Sunshine - Ko Samui

Ko Samui	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Ko Samui	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	6	8	8	7	6	5	5	5	5	5	6	6

Koh Samui, as other parts of Thailand, can be affected by tropical cyclones, in particular typhoons coming from the east, which in recent decades have entered the Gulf of Thailand only from mid-October to about 20 December, although being the sea always warm, it can not be excluded that they can also be formed in other periods. Anyway, the Gulf of Thailand is a relatively sheltered area, lying halfway between the Bay of Bengal, where cyclones are formed, and the South China Sea, where typhoons arrive from the east, and normally pass to the north, directly affecting mainland Thailand.



For swimming, the sea is warm all year round, as you can see in the following table.
Sea temperature - Ko Samui

Ko Samui	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Ko Samui	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	28	28	29	30	30	30	29	29	29	29	29	28
Sea (°F)	82	82	84	86	86	86	84	84	84	84	84	82

The best time to visit Koh Samui goes from mid or late January to April, with a preference for February, because it is a bit cooler than March and April.
In the summer, in July and August, it may very well not rain for a week, while at other times it can rain quite often, albeit in the form of showers, which then give way to the sun; however, the heat is muggy. The worst time is that of the retreating monsoon, especially the month of November.