Month by month
| Month | Afternoon | Sea | Rain | Verdict |
|---|---|---|---|---|
| January | 24.9 °C | 28.1 °C | 17 days | Possible, with trade-offs |
| February | 25.5 °C | 28.3 °C | 12 days | Ideal — one of the best times of year |
| March | 26.9 °C | 29 °C | 10 days | Ideal — one of the best times of year |
| April | 27.9 °C | 30.3 °C | 14 days | Possible, with trade-offs |
| May | 28 °C | 30.7 °C | 19 days | Unfavourable season |
| June | 28 °C | 30.4 °C | 16 days | Possible, with trade-offs |
| July | 27.5 °C | 29.9 °C | 18 days | Possible, with trade-offs |
| August | 27.6 °C | 29.8 °C | 18 days | Possible, with trade-offs |
| September | 26.6 °C | 29.5 °C | 20 days | Unfavourable season |
| October | 25.7 °C | 29.3 °C | 27 days | Unfavourable season |
| November | 25.2 °C | 29 °C | 26 days | Unfavourable season |
| December | 24.7 °C | 28.2 °C | 20 days | Unfavourable season |
Nearby in September
Before you go
Three things to check once you have picked your dates.
Entry
No visa needed for stays of up to 30 days. Passport valid for at least 6 months. Complete the digital arrival card online within 3 days before arrival; you may be asked to show 10,000 baht per person and onward travel; 1 blank page.
For a British passport, per the UK Foreign Office on 8 September 2026. Rules change: check before booking.
Safety
Outside the areas advised against
Parts of the country are advised against (Pattani, Yala and Narathiwat provinces and the south of Songkhla province (deep south); within 20 km of the Cambodian border); Koh Samui is not one of them.
Official advice updated 8 September 2026
Health
Vaccines · typhoid recommended for most travellers; hepatitis A and hepatitis B if you are not already vaccinated; Japanese encephalitis and chikungunya depending on activities and areas visited. Routine vaccines up to date (measles in particular).
Yellow fever · No yellow fever locally, vaccine not recommended. Certificate required only if you arrive from a country with yellow fever risk.
Malaria · Present. Areas: Primarily the provinces that border Burma (Myanmar), Cambodia (rare cases in Buri Ram Province), and Malaysia (especially in the rural forest and forest-fringe areas of these provinces) Rare to few cases in other parts of Thailand, including the cities of Bangkok (the capital), Chiang Mai, and Chiang Rai, or on the islands of Koh Pha Ngan, Koh Samui, or Phuket No malaria transmission on the islands of Krabi Province (Koh Lanta, Koh Phi Phi, Koh Yao Noi, Koh Yao Yai) or in Pattaya City. Preventive medication recommended: Provinces that border Burma (Myanmar), Cambodia (except Buri Ram Province), and Malaysia: Atovaquone-proguanil, doxycycline, tafenoquine All other areas with malaria transmission (including Buri Ram province): No chemoprophylaxis recommended (insect bite precautions and mosquito avoidance only) — prescribed before departure.
Mosquitoes · dengue and Zika reported: protect yourself from bites.
Per the CDC (page reviewed 25 August 2026); see a travel clinic a few weeks before departure.
Beau Fixe is not a safety or health authority. These notes summarise dated public texts and link to their source: France Diplomatie (Licence Ouverte 2.0), Foreign, Commonwealth & Development Office (gov.uk, Open Government Licence v3.0), Centers for Disease Control and Prevention (Source: CDC). Only the competent authorities are authoritative.
FAQ
Can you swim in Koh Samui in September?
The sea averages 29.5°C in September — yes, pleasant swimming.
How hot is Koh Samui in September?
Expect 26.6°C in the afternoon and 22.7°C at night, averaged over the last 10 years.
Does it rain a lot in Koh Samui in September?
Around 20 days with rain (≥ 1 mm) in the month — 68% of days.