Koh Samui in September
In September, Koh Samui averages 29°C (84°F) during the day and 26°C (78°F) at night, with frequent rain — about 29 days of the month see precipitation (172 mm total).
ShoulderConditions are warm and beach-friendly. Expect daily rain or showers, often in heavy bursts.
September offers comfortable temperatures in Koh Samui, with some rain to plan around.
How September compares to the rest of the year in Koh Samui
September is in the middle of the year for warmth in Koh Samui (7th of 12) and one of the wettest months (3rd of 12). Daytime temperature matches the city's annual average. September runs 1°C cooler than August (30°C) and cools 1°C in October (28°C).
September sunrise & sunset in Koh Samui
Times shown for Koh Samui local time (Asia/Bangkok), calculated for the 15th of September. They shift by a few minutes day to day across the month.
Photography light in Koh Samui — September
Golden-hour and blue-hour windows for the 15th of September, in Koh Samui's local time. Use these for soft warm light (golden) or cool architectural twilight (blue).
📅 When to book — Koh Samui in September
Travel writers usually only tell you when to go. Here's when to book flights and lodging for the best mix of price and availability, based on September's tier (Shoulder) and Koh Samui's popularity.
Crowds & pricing — September in Koh Samui
Koh Samui is in a shoulder season month — fewer crowds, slightly lower prices, and a calmer atmosphere than peak weeks. Most major sights stay open with normal hours, though a handful of seasonal restaurants or boat trips may run reduced schedules. Many regulars consider these months the best time to visit because you trade a small amount of weather reliability for a much more pleasant on-the-ground experience.
Sun & mosquito risk — September in Koh Samui
UV is a clear-sky sea-level estimate — clouds, altitude, and reflective surfaces (snow, water) shift it ±30%. Mosquito risk is climate-derived only; for current disease advisories (dengue, malaria, zika) check the CDC or WHO travel pages before your trip.
Sea temperature in Koh Samui — September
Across the year Koh Samui's sea swings from 28.6°C (January) to 32.1°C (May) — September sits at 30.1°C.
Koh Samui in other months
| Month | Avg high | Avg low | Rainfall | Rain days | Snow days |
|---|---|---|---|---|---|
| January | 28°C / 82°F | 25°C / 76°F | 113 mm | 24 | — |
| February | 28°C / 83°F | 25°C / 77°F | 33 mm | 17 | — |
| March | 30°C / 86°F | 26°C / 78°F | 32 mm | 17 | — |
| April | 31°C / 87°F | 27°C / 80°F | 107 mm | 23 | — |
| May | 31°C / 87°F | 27°C / 80°F | 130 mm | 29 | — |
| June | 30°C / 86°F | 27°C / 80°F | 96 mm | 27 | — |
| July | 30°C / 86°F | 26°C / 79°F | 115 mm | 28 | — |
| August | 30°C / 86°F | 26°C / 79°F | 94 mm | 28 | — |
| September | 29°C / 84°F | 26°C / 78°F | 172 mm | 29 | — |
| October | 28°C / 83°F | 25°C / 77°F | 234 mm | 31 | — |
| November | 28°C / 82°F | 25°C / 77°F | 274 mm | 29 | — |
| December | 28°C / 82°F | 25°C / 76°F | 182 mm | 24 | — |
Other cities with similar September weather
🌍 See the full ranked list of climate twins for Koh Samui in September →
Frequently asked questions
How hot is Koh Samui in September?
In September, Koh Samui averages a high of 29°C (84°F) and a low of 26°C (78°F) — generally warm.
Does it rain a lot in Koh Samui in September?
September is quite rainy in Koh Samui, with about 29 rainy days on average.
Is September a good time to visit Koh Samui?
September is warm but on the rainy side. It suits travelers who are comfortable with warm weather; check the month-by-month ranking to compare it against the rest of the year.
What should I pack for Koh Samui in September?
Pack lightweight, breathable clothing, sunglasses and sunscreen, plus a compact umbrella or rain jacket. See the full September packing list for Koh Samui for a complete, climate-based checklist.
Check the live 10-day forecast and sunrise/sunset.