Weather in Thailand in September: where it is dry and where the rainy season is
September divides Thailand into two different coasts. Koh Samui is still dry and sunny, while Phuket and Pattaya collect most of the month's rainfall. It is the same month, but the vacation experience turns out completely different.
We cross-checked long-term averages for three popular destinations (1991-2020, Thailand climate overview). Rainfall at the three resorts differs by almost three times. Let's look at each coast separately.
September in Thailand: in short
💡 In short:
- Koh Samui: 120 mm of rain over 11 days and 5 hours of sunshine per day (1991-2020 averages, Koh Samui data).
- Phuket: 325 mm over 19 days, only 3.5 hours of sun per day, but there are waves (1991-2020 averages, Phuket averages).
- Pattaya: 205 mm over 17 days, only Phuket is rainier (1991-2020 averages, Pattaya averages).
- Climate and water: 1991-2020 averages (Thailand summary averages).
The key difference lies in geography. Koh Samui sits in the Gulf of Thailand, while Phuket is washed by the Andaman Sea. The southwest monsoon hits these coasts differently, so the rains do not arrive at the same time. Pick the coast that matches your dates, and the weather will not ruin your trip.
Koh Samui weather in September
Koh Samui remains the calmest option in September. Air temperatures range from 24.8 to 31.9 °C, and the monthly average stays at 28.4 °C (1991-2020 averages, Koh Samui climate). Rain here is brief and usually arrives toward evening.
© Ssuri · CC BY-SAPhoto: Ssuri, CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0), Wikimedia CommonsThe first half of the month is noticeably sunnier. Koh Samui gets more sun than the other two resorts (same climate normals). The water near the shore is calm, and swimming is comfortable almost every day.
- The catch: by late September the sea becomes rough, so if you are traveling with children, it is better to come in the first half of the month.
Heavy downpours reach Koh Samui only in October and November, when rainfall rises to 295 and 445 mm per month (1991-2020 normals, monthly statistics for Koh Samui). If you are flying in early autumn, choose Koh Samui.
Phuket weather in September
Phuket in September is still under the wet monsoon. The island receives 325 mm of rainfall during the month, with rain on about 19 days (1991-2020 normals, Phuket weather by month). The sun shows itself for only 3.5 hours a day.
© JULIANISME · CC BY-SAPhoto: JULIANISME, CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0), Wikimedia CommonsAir temperature ranges from 23.8 to 31.4 °C, with the average at 27.6 °C (1991-2020 normals, Phuket temperature and rainfall). The sea is warm all year round, and in September the water is around 29 °C. You can swim almost every day, but the sky is often overcast in the mornings.
- The catch: almost 19 rainy days a month and the least sun of the three resorts.
September in Phuket has its own advantage. Waves on the western beaches grow, and surfers come specifically for that. It is also the low season, so accommodation prices drop. Rain comes in waves and usually does not last all day.
Pattaya weather in September
For rainfall, Pattaya in September is second only to Phuket. The city receives 205 mm of rainfall over 17 days (1991-2020 normals, Pattaya weather by month). The sun shines for about 3.5 hours a day.
© Vyacheslav Argenberg · CC BYPhoto: Vyacheslav Argenberg, CC BY (https://creativecommons.org/licenses/by/4.0), Wikimedia CommonsAir stays in the 25.2-31 °C range, water remains warm, around 29 °C (1991-2020 normals, water temperature in Pattaya). One plus: downpours are short and don't break the heat. The catch is that after heavy rain the sea gets murky and swimming loses its point.
September, along with May and October, is one of the rainiest months in Pattaya. For a beach vacation this is not the best choice. But the city has plenty of indoor entertainment for bad weather, so you won't get bored.
Comparing resorts in September
Resort | Air, °C | Water, °C | Precipitation, mm | Rainy days | Sun, h/day | Source |
|---|---|---|---|---|---|---|
Samui | 28.4 | 29 | 120 | 11 | 5 | |
Phuket | 27.6 | 29 | 325 | 19 | 3.5 | |
Pattaya | 28.1 | 29 | 205 | 17 | 3.5 |
If you compare Samui, Phuket, and Pattaya by September rainfall, the difference is almost threefold: 120 mm versus 325 mm in the same month. Table data: 1991-2020 average normals (data source).
Thailand on the map
If you are choosing between coasts, start with the map. It will show how far the resorts are from each other and where the monsoon boundary runs.
⁉️🤔 Frequently asked questions
Can you swim in Thailand in September?
Yes, the sea is warm at all three resorts. You can swim every day, but waves and rain are stronger in Phuket and Pattaya. After a downpour the water near the shore gets murky, so go into the sea in the morning (1991-2020 averages).
Where in Thailand should you go with children in September?
With children, choose Samui. It has less rain and more sun than Phuket and Pattaya. The water near the shore is calm, especially in the first half of the month (1991-2020 normals, Samui climate data).
What is the water temperature in September?
Water stays around 29 °C in Samui, Phuket, and Pattaya, so swimming is comfortable without a wetsuit. Air is on average cooler than water: from 27.6 to 28.4 °C. At night the thermometer drops only a few degrees (1991-2020, water temperature normals).
When does the rainy season end?
On Phuket the monsoon calms down starting in November, and the driest period falls in December and January. In Pattaya the rains stop by mid-November. On Samui it is different: heavy downpours arrive in October and November, so it is better to go there in September (1991-2020 normals, seasonal data by region).
Is it expensive to vacation in Thailand in September?
September is part of the low season, so hotels offer discounts. On Phuket it is the cheapest month of the year. There are fewer tourists both on the beaches and on excursions. You get the savings together with rainy weather, which is worth keeping in mind.
In short: where to go in September
September in Thailand can be very different. If you want dry days and a calm sea, choose Samui. If you are ready for rain for the sake of surfing and low prices, go to Phuket. Pattaya suits those who are going for the city, not for the beach.
- If you have a vacation with children: Samui.
- If you want surfing and a budget: Phuket.
- If you have a short trip and want the city: Pattaya.
Before buying tickets, check the forecast for your dates: the weather varies noticeably from year to year.
A small editorial team writing guides for independent travel. We travel ourselves, but we haven’t been everywhere, so we check prices, opening hours and how to get there against official sources.