Thailand
Koh Samui in September: weather, sea & travel advice
This month's data profile
Rain shapes the decision
Koh Samui in September is climatically well suited to beach holidays. The average daily high is around 29 °C, and the sea is about 29 °C. With about 27 rainy days it is very wet.
Historical monthly averages, not a weather forecast. Individual days and years can differ substantially.
Rank among 12 monthly averages
56/100 · #7
Warmth
#7
Dryness
#11
Sun
#7
Sea warmth
#6
Published best categories: none
Koh Samui · SeptemberPhoto: Sonia Selivonchik · CC BY-SA 4.0Climate facts
- Average high
- 29 °C
- Average low
- 27 °C
- Sea
- 29 °C
- Rainy days (est.)
- 27 rainy days (est.)
- Sun (est.)
- 6 h/day
- Humidity
- 81 %
For your trip planning
September: wetter than August, clearly drier than October
In our series, September on Koh Samui keeps August's average highs and lows but brings somewhat more precipitation and slightly fewer estimated sun hours. October is the bigger shift, with clearly more rain. September ranks fourth wettest of the year. The sea stays warm, so swimming is plausible if you plan beach time flexibly around showers.
Climate evidence with direct month comparison
The highlighted row is the selected month. Precipitation total and rainy days are separate monthly averages.
| Month | Avg high °C | Avg low °C | Rainy days (est.) | Precipitation mm | Sun h/day (est.) | Humidity % | Wind m/s | Sea °C | Score |
|---|---|---|---|---|---|---|---|---|---|
| August | 29 | 27 | 27 | 163 | 6.5 | 81 | 4.4 | 29 | 55 |
| September (selected) | 29 | 27 | 27 | 181 | 6.3 | 81 | 4.0 | 29 | 56 |
| October | 28 | 27 | 28 | 265 | 5.6 | 82 | 3.4 | 29 | 56 |
How do the previous and next months compare?
Baseline: September (56/100)
Positive and negative values show the change from the selected month.
| Score Δ | Average high Δ | Average low Δ | Rainy days Δ | Rain Δ | Sun Δ | Humidity Δ | Wind Δ | Sea Δ | |
|---|---|---|---|---|---|---|---|---|---|
| August | -1 | +0.4 °C | +0.2 °C | -0.4 | -17.7 mm | +0.2 h | -0.3 % | +0.4 m/s | +0.2 °C |
| October | ±0 | -0.4 °C | ±0 °C | +1.3 | +84.3 mm | -0.7 h | +0.5 % | -0.6 m/s | -0.2 °C |
All twelve months in context
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Frequently asked questions
How warm is Koh Samui in September?
The average daily high is around 29 °C, with an average daily low of about 27 °C.
Can you swim in Koh Samui in September?
The sea is about 29 °C (very warm).
Does it rain a lot in Koh Samui in September?
The curated estimate is about 27 rainy days per month, not a directly measured long-term average. Total precipitation averages around 181 mm for the month. A rainy day does not mean all-day rain.
When is the best time to visit Koh Samui?
By the climate score, January, February, March and December are the strongest months for Koh Samui. For a beach trip: January, February, March and December. September is not among them.