When to Visit Koh Samui

Koh Samui's seasons are not the same as Phuket's. Thailand has two coasts with almost opposite rainy seasons: the Andaman coast (Phuket) is wettest about May to October, while Koh Samui and the Gulf islands get most of their rain from the northeast monsoon, roughly October to December. Daytime temperatures stay in a narrow warm band of about 29-32°C all year, and the sea stays around 28-30°C.

The year in four parts. January to April is the dry and cooler season: February and March are typically the driest and sunniest months, with calm, clear water - the most popular and most expensive period. March to May is the hot season: April is usually the hottest month (highs around 32-34°C) and hosts Songkran, the Thai New Year water festival, in mid-April. May to September is a warm shoulder period: mostly sunny mornings with short heavy showers rather than all-day rain, greener landscapes and prices roughly 20-30% below peak - and often drier than Phuket at the same time. October to December is the rainy season: November is normally the wettest month (long-term average over 400 mm), rain can come as multi-day grey spells, seas get rough especially on the east coast, ferries and boat trips are sometimes cancelled, and Ang Thong Marine Park usually closes from 1 November (2026 reopening date not yet confirmed)^[1].

Best time by goal. Best all-round weather: February and March. Best value with still-good weather: May, June, July. European summer holidays (July-August): Samui is usually a better bet than Phuket. Christmas and New Year: weather usually turns good by mid-December, but it is the busiest, priciest fortnight - book early. Snorkelling and boat trips: January to June. Avoid October-November if weather matters.

Two weather quirks worth knowing. Tides: the Gulf often has only one tide cycle a day, and around May-September the sea is seasonally lower, so flat north- and west-coast beaches can be very shallow at low tide. Storms: serious tropical storms are uncommon but do happen - follow local warnings. Monthly figures in the weather category are long-term averages, not promises.

1. [1] Tourism Authority of Thailand — national park seasonal closures · checked 2026-11-10