Koh Samui FAQ: Weather

Reviewed by the Samui Local Hub team · Updated 25 Sept 2026

Main page on this topic: Weather by month

What is the rainy season like in Koh Samui?

Rainy season on Koh Samui is roughly October to December, driven by the northeast monsoon - different from Phuket, which is wettest May to October. November is usually the wettest month, and rain can come as multi-day grey spells rather than short showers. Seas can be rough, ferries and boat trips may be cancelled, and Ang Thong Marine Park usually closes from 1 November (2026 reopening date not yet confirmed)^[1]. The upside: the lowest prices and fewest crowds of the year.

Is November a good time to visit Koh Samui?

Only if weather does not matter much to you. November is normally the wettest month of the year on Koh Samui (long-term average over 400 mm of rain), with possible multi-day rain, rough seas, and frequent ferry and boat-trip disruption. Ang Thong Marine Park is usually closed from 1 November (2026 reopening date not yet confirmed). Prices are the lowest of the year. If you must travel in November and want dry weather, Phuket and the Andaman coast are usually the better choice that month.

How warm is the sea in Koh Samui?

The sea around Koh Samui stays warm all year, roughly 28-30°C. The warmest sea is around April (about 30°C) and the coolest around December-February (about 28°C) - still bath-warm by most standards. You will never need a wetsuit for swimming. For snorkeling, underwater visibility matters more than temperature: it is usually best in the calm, dry months from about January to June.

Is Koh Samui good in July and August?

Yes, often better than Phuket at that time of year. Koh Samui sits in the Gulf of Thailand and is partly sheltered from the southwest monsoon that soaks the Andaman coast in mid-year. Expect warm, bright mornings with brief afternoon showers and mostly calm seas. July and August are busy with European family holidays, so book family accommodation early, but weather-wise this is a reasonable and underrated window.