Destination guide
Koh Samui weather by month
February is the best time to visit Koh Samui for relatively dry weather, with January and March also good choices. Daytime temperatures stay around 28°C to 30°C throughout the year, so rain and sunshine matter more than warmth when choosing your month. February averages just 6 rainy days. The catch is that even the driest period is not rain-free, while the wetter months suit travellers who can keep outdoor plans flexible.
- Hottest month
- 30.1°CMay
- Coolest month
- 27.5°CJanuary
- Warmest sea
- 30.5°CMay
- Flight from London
- 13 h 6 minLong-haul (7 h+) · estimated
Koh Samui, month by month
A little perspective
Koh Samui through the year
Every month at a glance; "best" marks our three top months on warmth, dryness and sunshine.
| Month | Day | Night | Sea | Sun/day | Rain mm | Rainy days | Days 25°C+ |
|---|---|---|---|---|---|---|---|
| Januarybest | 27.5°C | 25.4°C | 27.7°C | 5.1 | 78 | 10 | 100% |
| Februarybest | 28.2°C | 25.9°C | 28.2°C | 6.2 | 35 | 6 | 100% |
| Marchbest | 29.2°C | 26.6°C | 29.2°C | 6.2 | 87 | 9 | 100% |
| April | 30.0°C | 27.3°C | 30.3°C | 5.2 | 96 | 12 | 100% |
| May | 30.1°C | 27.3°C | 30.5°C | 2.7 | 119 | 22 | 100% |
| June | 29.9°C | 27.1°C | 30.0°C | 1.8 | 93 | 19 | 100% |
| July | 29.6°C | 26.8°C | 29.4°C | 1.7 | 88 | 19 | 100% |
| August | 29.4°C | 26.6°C | 29.2°C | 1.6 | 96 | 19 | 100% |
| September | 28.9°C | 26.3°C | 29.1°C | 1.6 | 143 | 23 | 100% |
| October | 28.1°C | 25.6°C | 29.1°C | 2.0 | 283 | 26 | 100% |
| November | 27.8°C | 25.6°C | 28.7°C | 3.2 | 281 | 21 | 100% |
| December | 27.5°C | 25.5°C | 28.0°C | 3.9 | 144 | 14 | 100% |
Day = average daytime high, night = average night-time low, 1991-2020.Sun is hours per day; rainy days count 1 mm or more.
Day, night and sea
Monthly average temperatures, 1991-2020
- Day
- Night
- Sea
Best months here
On warmth, low rainfall and sunshine, start with these.
When to go to Koh Samui
For beach weather, look at months where the daytime average clears 25°C and rainy days stay low: January, February, March, April, May, June, July, August, September, October, November, December. October is the wettest month (283 mm, about 26 rainy days) and February the driest (35 mm). The sea is warmest in May at 30.5°C and coolest in January at 27.7°C.
Getting there
A direct flight from London Gatwick to Samui International Airport (USM, 14 km from Ko Samui) takes roughly 13 h 6 min - long-haul (7 h+) by UK standards. Time-zone: Asia/Bangkok.
Records and extremes, 1991-2020
The hottest day recorded in Koh Samui between 1991 and 2020 reached 34.2°C on 17 April 2019; the coldest fell to 21.7°C on 29 January 2017. In May one day in ten tops 31.4°C and about 54% of days pass 30°C; in January one night in ten drops to 24.2°C or below. The wettest single month of the period was March 2011 with 942 mm, against a March average of 87 mm.
Sunrise, sunset and daylight in Koh Samui
Local time on the 15th of each month; daylight-saving rules as of 2020.
| Month (15th) | Sunrise | Sunset | Daylight | Sunshine/day |
|---|---|---|---|---|
| January | 06:41 | 18:18 | 11.6 h | 5.1 h |
| February | 06:40 | 18:29 | 11.8 h | 6.2 h |
| March | 06:27 | 18:31 | 12.1 h | 6.2 h |
| April | 06:10 | 18:30 | 12.3 h | 5.2 h |
| May | 06:00 | 18:33 | 12.6 h | 2.7 h |
| June | 06:00 | 18:41 | 12.7 h | 1.8 h |
| July | 06:07 | 18:45 | 12.6 h | 1.7 h |
| August | 06:12 | 18:38 | 12.4 h | 1.6 h |
| September | 06:10 | 18:21 | 12.2 h | 1.6 h |
| October | 06:08 | 18:04 | 11.9 h | 2.0 h |
| November | 06:14 | 17:55 | 11.7 h | 3.2 h |
| December | 06:28 | 18:02 | 11.6 h | 3.9 h |
January to March: the best balance of warmth and sunshine
Choose February if your priority is spending time outdoors with fewer rain interruptions. Its 6 rainy days compare with 10 in January and 9 in March, making it the clearest choice for a weather-led holiday. February and March also average 6 hours of sunshine a day, slightly more than January. January is the coolest month, but daytime temperatures of 28°C still call for light clothing rather than warm layers. March feels warmer, particularly after dark, with nights averaging 27°C. The sea stays warm throughout this period, so there is little reason to delay a swimming-focused trip for higher water temperatures.
April to August: hotter days, then less sunshine
April offers a warmer option before rain becomes much more frequent: expect daytime temperatures around 30°C, with 12 rainy days and 5 hours of sunshine. May is the hottest month, but its 22 rainy days make it a less straightforward choice for long outdoor days. June and July remain around 30°C, while August eases to 29°C. From June through August, 19 rainy days and just 2 hours of daily sunshine favour flexible plans over a holiday built around clear skies. Keep midday activity gentle and pack light, breathable clothes with rain protection. Nights around 27°C offer little cooling, and the warm sea will not feel sharply refreshing.
September to December: frequent rain, with improvement towards year end
September and October are difficult choices if dry outdoor time matters most. September averages 23 rainy days, rising to 26 in October, the wettest month. Both have just 2 hours of sunshine a day, despite daytime temperatures remaining warm. November brings some improvement, but 21 rainy days still call for plans that can change with the weather. December is a more useful compromise: rain days fall to 14 and sunshine rises to 4 hours, though it remains wetter than January to March. Pack rain protection without adding heavy clothing, as evenings stay warm. Sea temperatures remain inviting, but warmth alone does not tell you if swimming conditions are suitable.
A little more detail
Koh Samui weather FAQs
Plain answers, grounded in the averages.
What is the best time to visit Koh Samui?
Judged on warmth, low rainfall and sunshine, the best months in Koh Samui are February, March and January. May is the hottest month (average high 30.1°C) and January the coolest (27.5°C).
How many months a year is Koh Samui over 25°C?
12 months in Koh Samui average a daytime high of 25°C or more, and 12 average 20°C or more.
When is the rainy season in Koh Samui?
October is the wettest month in Koh Samui (283 mm on average) and February the driest (35 mm). Annual rainfall averages 1543 mm.
What is the hottest temperature recorded in Koh Samui?
The hottest day recorded in Koh Samui between 1991 and 2020 reached 34.2°C on 17 April 2019; the coldest fell to 21.7°C on 29 January 2017. Records come from daily data for 1991-2020, not from a single station's all-time history.
Is the sea warm enough for swimming all year?
Yes, sea temperatures range from 28°C to 30°C, so cold water is unlikely to be the deciding factor. Choose your month around rain and sunshine instead, and check local conditions before swimming rather than treating warm water as a sign of calm seas.
Will I need warm clothes in the evenings?
Outdoor evenings stay warm, with average night temperatures between 25°C and 27°C. Light, breathable clothing is the practical choice throughout the year. January has the coolest nights, but even then the average does not suggest chilly evening weather.
Is December as dry as February?
No. December averages 14 rainy days compared with February's 6, and it has less sunshine. December is an improvement on the wetter autumn months, but February is the better choice if your main aim is fewer weather interruptions.
See also: Thailand weather by month · where else is hot in May
About these averages
ERA5-Land climate reanalysis via Open-Meteo (11 km grid) and NOAA Coral Reef Watch 5 km sea temperatures, 1991-2020
Averages computed from daily values at Ko Samui. Sea temperature from the nearest 5 km sea cell, 1.7 km from that point. Sunshine hours are estimated from average cloud cover.Records are the highest and lowest daily values in that 30-year period, not all-time records. These are climate averages, not a forecast for your trip. How we measure holiday weather
- Hot · 25°C+
- Warm · 20°C to below 25°C
- Mild · 15°C to below 20°C
- Cool · below 15°C