The single most useful thing to know about Koh Samui’s weather is that it does not follow the pattern of the rest of Thailand. Phuket, Krabi and the Andaman coast are wettest from about May to October. Samui, on the other side of the peninsula in the Gulf of Thailand, stays largely dry through those months and gets its heavy rain later — from October to mid-December.
That makes Samui the best beach choice in Thailand for a July or August holiday, and a risky one in November.
The seasons at a glance
- Dry season — January to April. January can still bring showers and wind early in the month, but by February the weather is settled: sunshine, low humidity and calm seas. February to April is the most reliable period of the year.
- Hot season — April to June. The hottest months, with afternoon temperatures in the mid-thirties and little wind. The sea is flat and clear, which is ideal for snorkelling and diving trips.
- Summer — July to September. Mostly fine with occasional short, heavy showers, usually late in the day. This is a second peak season, driven by European school holidays, and prices rise in July and August.
- Rainy season — October to mid-December. The north-east monsoon brings the island’s heaviest rain. November is normally the wettest month, with frequent downpours, rough seas and occasional flooding on low-lying roads. It does not rain all day every day, but several grey days in a row are common.
Temperatures hardly change: expect roughly 25–33°C all year, with sea temperatures around 28–30°C.
Month by month
| Month | Weather | Sea & boat trips | Crowds & prices |
|---|---|---|---|
| January | Improving quickly; some showers and wind early on | Can still be choppy; settling later in the month | Peak season, high prices |
| February | Dry, sunny, less humid | Calm | Busy; Chinese New Year can add crowds |
| March | Dry and getting hotter | Calm and clear | Moderate |
| April | Hot; Songkran water festival mid-month | Excellent | Busy around Songkran |
| May | Hot, with a few showers | Very good | Quieter, good value |
| June | Mostly fine, occasional showers | Very good | Quieter, good value |
| July | Mostly fine, breezy | Good | Second peak; book ahead |
| August | Mostly fine, some showers | Good | Second peak; book ahead |
| September | More unsettled, more showers | Usually fine | Quiet, good value |
| October | Rain increasing, thunderstorms | Becoming rough; some cancellations | Low season |
| November | Wettest month; heavy rain, possible flooding | Often rough; Ang Thong normally closed | Lowest prices |
| December | Wet until mid-month, then improving | Rough early, better by Christmas | Peak prices from about 20 December |
When boat trips run — and when they do not
Ang Thong National Marine Park normally closes to visitors for part of the monsoon, roughly from 1 November until a few days before Christmas. The exact dates are announced each year by the park authorities. Outside the closure, trips to Ang Thong, Koh Tao and Pig Island run daily but are cancelled whenever the sea is unsafe, which is most often between October and December.
Underwater visibility is generally best from about March to September. If diving or snorkelling is the point of your trip, come then.
Is the rainy season worth considering?
It can be. Hotel rates in October and November are the lowest of the year, the island is green, the waterfalls are in full flow and there are no crowds. But you must be relaxed about losing days to rain, boat trips may not run, and the east-coast beaches can be too rough for swimming. If beach weather matters, choose the Andaman coast (Phuket, Krabi, Koh Lanta) in November and early December instead — its dry season is just starting.
Festivals and events
- Chinese New Year (January or February) is celebrated in Nathon, Maenam and Hua Thanon, where many families have Hainanese roots.
- Songkran, the Thai New Year, falls on 13 April on Samui: expect to be soaked in island-wide water fights.
- Loy Krathong (usually November) sees candle-lit floats launched on the water, notably at Chaweng Lake and the temples.
- The Full Moon Party on Koh Phangan takes place every month; ferries and hotels on both islands fill up around it.
