Weather in Ko Samui in September 2025
Average weather
Average temperature in Ko Samui in September
The temperature in September is +27°C on average, with a range between a maximum of +28°C during the day and a minimum of +24°C at night.
The average maximum temperature goes up 25% by the end of the month, compared with the beginning of the month, from +26°C to +27°C.
Is Ko Samui warm or cold in September?
Ko Samui is typically warm in September.
Maximum temperatures go up compared to the previous month from +27°C to +27°C. The minimum temperature also goes down from +25°C to +24°C.
It’s generally warm, but pack a long-sleeve shirt for evenings.
September is one of the warmest months in Ko Samui, here is the comparison with other months:
Water temperature in Ko Samui in September
Water in Ko Samui in September is Very warm. It varies from +28°C to +30°C, with an average of +29°C. Note that the air temperature is lower than the water temperature, averaging around +27°C.
In September, water temperatures are average relative to the rest of the year:
Can you swim in Ko Samui in September?
Climate in Ko Samui in September
| August | September | October | ||||
|---|---|---|---|---|---|---|
| Avg. temperature | ||||||
| +27°C | +27°C | +26°C | ||||
| Min. temperature | ||||||
| +20°C | +21°C | +21°C | ||||
| Max. temperature | ||||||
| +33°C | +32°C | +32°C | ||||
| Rainfall | ||||||
| 85 mm | 142 mm | 242 mm | ||||
| Wind | ||||||
| 3 m/s | 3 m/s | 2 m/s | ||||
| Avg. water temperature | ||||||
| +29°C | +29°C | +29°C | ||||
| Humidity | ||||||
| 78% | 79% | 82% | ||||
| Air pressure | ||||||
| 1008 HPa | 1008 HPa | 1009 HPa | ||||
| Sunny days | ||||||
| 0 | 0 | 0 | ||||
| Cloudy days | ||||||
| 4 | 2 | 2 | ||||
| Rainy days | ||||||
| 18 | 17 | 16 | ||||
| Snowy days | ||||||
| 0 | 0 | 0 | ||||
September weather is still as ok as in August. The temperature levels stay the same in the area of +27°C for average maximum temperature during the day. Prepare for more rain. Average rainfall goes from 85 mm to 142 mm. Humidity rises from 78% to 79%. The average wind speed doesn’t change much during September, staying around 3 m/s. The amount of sunny days stay about the same, averaging 0 sunny days during the month.
October brings much the same weather, it remains ok.
Wind
The average wind speed is 3 m/s getting up to 5 m/s which is a light breeze; barely moving the tree leaves.
The average wind speed in Ko Samui is gradually decreasing during September, going from 5 m/s to 1 m/s over the course of the month.
Rain
How was the weather last September in Ko Samui?
Climate Change over Years
The climate in Ko Samui in September has not changed much over the last 43 years.
The average temperature was the lowest in 1986 (+26°C) and the highest in 2012 (+28°C). The driest year was 1997 with only 927 mm of rainfall, and the wettest year was 1999 with 1981 mm.
FAQ
Is September a good time to visit Ko Samui?
Can you sunbathe in Ko Samui in September?
Can you swim in Ko Samui in September?
Is Ko Samui rainy in September?
Is it sunny in Ko Samui in September?
Other cities in Thailand
Monthly weather in Ko Samui
Source of the data
For this page, we’ve looked at the typical weather in Ko Samui, based on historical hourly weather data from January 1, 1979, to December 31, 2023.
For past dates, we show actual recorded data at the location at that time. Forecasts are based on recorded weather during that day of the year, averaged over the total years for which we have records (unless stated otherwise). The daily temperature is the highest recorded temperature in the shadow during that day.