Koh Samui sits in the Gulf of Thailand on Thailand's east coast — and this geographical fact defines everything about its weather. Unlike Phuket and Krabi on the western coast, where the dry high season runs from November to April, Samui is shaped by the northeast monsoon rather than the southwest one. This means the island's seasons are shifted compared to the "classic" Thai weather calendar.

The result is that Samui enjoys excellent weather in summer — precisely when much of the rest of Thailand is being drenched. From May through September, Samui has hot sun, blue skies, and warm, clear seas. November, by contrast, is the peak of Samui's rainy season, averaging around 490 mm of rainfall in a single month.

Koh Samui weather month by month

January–March: transition to high season

After the November rains, the sea gradually calms. January can still be wet (around 138 mm, 11 rainy days), but by February rainfall drops significantly (58 mm, roughly 5 rainy days). March is nearly dry: 78 mm, about 5 rainy days. Air temperatures range 25–31°C, and the water is warm and crystal clear. February and March are among the very best months to visit Samui: the sea is calm, the weather is comfortable, and there are fewer tourists than in the November–December period.

April–May: hot transitional months

April is the hottest month, with daytime temperatures reaching 32°C and very little rain (77 mm). May sees increasing humidity, with rainfall climbing to around 147 mm spread across approximately 15 days. Rain showers are typically brief — a powerful tropical downpour that passes within an hour or two and returns the sunshine.

June–September: best value-for-weather season

While Phuket and Krabi are battered by the southwest monsoon, Samui basks in sunshine. Monthly rainfall of 113–123 mm consists mostly of evening thunderstorms that clear by morning. The sea is warm, there are fewer tourists, and villas and hotels are noticeably cheaper. June through September is ideal for those seeking good weather without peak-season crowds or prices.

October–December: rainy season

October (290 mm), November (490 mm), and December (209 mm) mark the northeast monsoon on Samui. This does not mean non-stop rain: tropical downpours can be intense but are usually short-lived. However, the sea can become rough during this period, particularly on the north and east coasts. For surfers, it is actually the most exciting time of year. For beach holidays, the south and west coasts offer more sheltered conditions.

Summary: when is the best time to visit Koh Samui?

  • Best months: February, March, April, June, July, August
  • Good with caveats: January, May, September
  • Rainy season: October, November, December — not ideal for beach holidays

It is worth remembering that Samui has no truly "bad" weather in any absolute sense. Even in the rainiest month you can have brilliant sunny days, and even in the driest month unexpected showers are possible. The climate is tropical — surprises are part of the experience — but it is precisely this that gives every Samui trip its own character.