Thailand Koh Samui Weather in September — Climate Guide
September in Koh Samui is moderate
Koh Samui Weather in September — Quick Answer
As of 2026- Avg temp
- 31° / 25°
- Rainfall
- 150mm · 15 days
- Crowds
- Moderate
As of 2026, Koh Samui in September averages a high of 31°C and low of 25°C with 150mm of rain over about 15 days. Rains building toward the wet peak — warm, humid, with more frequent showers. Around 31°C/25°C. Light clothing, swimwear, SPF 50+, and a reliable rain layer/umbrella. Sea ~29°C.
September is moderate for Koh Samui.
September is a transition month as Koh Samui's rains build toward the October-December wet peak. It averages 31°C/25°C with rainfall rising (around 150mm) and more frequent, sometimes heavier showers.
It's a quieter, better-value time — prices and crowds drop after the European summer — and there are still plenty of sunny spells, but the weather is less dependable than the June-August window, and boat trips to Ang Thong and the outer islands can be affected by rougher seas.
Travel score
Koh Samui September Weather Stats
Avg High
31°C
Avg Low
25°C
Rainy Days
15
150mm total
Sunshine
6h
Daily avg
Humidity
80%
muggy
UV Index
10
Very high
Wind
12 km/h
Gusts 24
Sea Temp
29°C
Avg water temp
Crowds
Moderate
At attractions
September Sunrise & Sunset
Air Quality
PM2.5
14 μg/m³
PM10
26 μg/m³
Status
good
Feels-Like Temperature & Wind
Feels like
Daytime
37°C
Nighttime
27°C
Humidity and wind affect how the actual temperature feels.
Wind
12 km/h
Max gust 24 km/h
Activities Recommended for September
September Travel Pros & Cons
Pros
- Shoulder-season value — quieter and cheaper
- Still sunny spells between showers
- Lush, green landscapes
- Good for spa-and-food-focused trips
Cons
- Rains building — less reliable weather
- Boat trips less dependable (rougher seas)
- High humidity
- Approaching the wettest months
September Events & Festivals
- Rains increasing ahead of the Oct-Dec wet peak
- Shoulder-season value — quieter and cheaper
- Boat trips still possible but less reliable
- Lush, green landscapes
Koh Samui in September — Full Picture
September is a transition month as Koh Samui's rains build toward the October-December wet peak. It averages 31°C/25°C with rainfall rising (around 150mm) and more frequent, sometimes heavier showers.
It's a quieter, better-value time — prices and crowds drop after the European summer — and there are still plenty of sunny spells, but the weather is less dependable than the June-August window, and boat trips to Ang Thong and the outer islands can be affected by rougher seas.
The landscape is lush and green, and a rainy-season trip can still work well with a flexible plan that mixes beach time with indoor options (spa, food, Fisherman's Village). Just don't count on guaranteed sunshine.
The sea is warm (~29°C). UV stays high (10) even under more cloud.
If reliable beach-and-boat weather matters, the dry season (Jan-Apr) or the June-August spell are safer bets than September, which sits on the edge of the wet peak.
What to Wear & What to Pack
Rains building toward the wet peak — warm, humid, with more frequent showers. Around 31°C/25°C. Light clothing, swimwear, SPF 50+, and a reliable rain layer/umbrella. Sea ~29°C.
Packing Checklist
Seasonal Weather Overview
Spring (Mar·Apr·May)
31.7°C
25°C
10/mo
Summer (Jun·Jul·Aug)
32°C
25°C
13.3/mo
Autumn (Sep·Oct·Nov)
Current30°C
24.3°C
18/mo
Winter (Dec·Jan·Feb)
29°C
24°C
12.3/mo
Koh Samui Monthly Weather Comparison
| Month | High | Low | Rain | Sun | Humid | UV | Crowd | Price |
|---|---|---|---|---|---|---|---|---|
| 29° | 24° | 14d·180mm | 6.5h | 82% | 10 | Very busy | Peak | |
| February (Best) | 30° | 24° | 6d·60mm | 8h | 78% | 11 | Very busy | Peak |
| March (Best) | 31° | 25° | 7d·75mm | 8.5h | 78% | 12 | Busy | Peak |
| 32° | 25° | 9d·100mm | 8.5h | 78% | 12 | Busy | Peak | |
| 32° | 25° | 14d·150mm | 7h | 80% | 11 | Moderate | Shoulder | |
| 32° | 25° | 13d·110mm | 7.5h | 78% | 11 | Busy | Shoulder | |
| 32° | 25° | 13d·110mm | 7.5h | 78% | 11 | Very busy | Peak | |
| 32° | 25° | 14d·110mm | 7h | 78% | 11 | Very busy | Peak | |
| 31° | 25° | 15d·150mm | 6h | 80% | 10 | Moderate | Shoulder | |
| 30° | 24° | 19d·290mm | 5h | 84% | 9 | Quiet | Low season | |
| 29° | 24° | 20d·340mm | 4.5h | 85% | 9 | Quiet | Low season | |
| 28° | 24° | 17d·230mm | 5h | 84% | 9 | Very busy | Peak |
View Other Months
Koh Samui September Hotel Picks
Frequently Asked Questions
Is September a good time to visit Koh Samui?
Koh Samui in September is moderate. Tourist crowds are moderate and flight/hotel prices are shoulder.
What's the average temperature in Koh Samui in September?
Koh Samui September averages a high of 31°C and a low of 25°C.
Does it rain much in Koh Samui in September?
Koh Samui September averages 150mm of rainfall over about 15 rainy days.
What should I wear in Koh Samui in September?
Rains building toward the wet peak — warm, humid, with more frequent showers. Around 31°C/25°C. Light clothing, swimwear, SPF 50+, and a reliable rain layer/umbrella. Sea ~29°C.
What time are sunrise and sunset in Koh Samui in September?
Koh Samui September sunrise is 06:00 and sunset is 18:20, with 12h 20min of daylight.
What's the air quality like in Koh Samui in September?
Koh Samui September air quality (AQI) averages 38, rated Good. PM2.5 averages 14μg/m³.
What activities are recommended in Koh Samui in September?
In September, Koh Samui is great for Beach days between showers, Ang Thong/Koh Tao (check conditions), Spa days + Fisherman's Village.
Is September peak season in Koh Samui?
Koh Samui September crowds are moderate and prices are shoulder. You can travel relatively comfortably without long queues.
Best Months to Visit Koh Samui
View Other Months
Planning a September trip to Koh Samui?
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Why you can trust weather guide
Based in Chiang Mai for 8+ years, with 30+ countries visited across Southeast Asia, Japan, and Europe. Every detail in this guide is primary-source verified as of April 2026, with prices auto-refreshed via live exchange rate APIs. This isn't AI-generated boilerplate — it's written from the perspective of someone who has actually been there.