Weather in Koh Samui in September

How September compares

September is the 7th warmest month of the year in Koh Samui and ranks 36th of 134 destinations we compare for September - see where else is hot in September. Within Thailand it is 3rd of 6 (Thailand in September). Against London's average high of 18.6°C in September, Koh Samui is 10.3°C warmer by day and 15.1°C warmer at night.

Daytime and night-time in September

The typical September day in Koh Samui peaks at 28.9°C, but that is an average: the warmest 10% of days reach 29.8°C or more. About 100% of days top 25°C and 1 in 11 pass 30°C. Nights average 26.3°C; one night in ten drops to 25.3°C or lower, so pack a layer for evenings. Between 1991 and 2020 the hottest September day here reached 32.1°C (2018-09-01) and the coldest night 23.9°C (2020-09-28).

Rain in September

Wet (more than 8 rainy days): about 23 days with 1 mm or more, 143 mm in total. Year to year it varies a lot - the wettest September was 2015 with 242 mm, the driest 2002 with 72 mm. Humidity averages 80%, which makes the heat feel heavier than the thermometer suggests.

Sea temperature in September

The sea averages 29.1°C - warm enough to swim (22°c+). Depending on the year it ranges from 28.7°C to 29.6°C. For the warmest sea here, come in May (30.5°C).

Sunshine and daylight

Around 1.6 hours of sunshine a day (estimated from cloud cover), 12.0 hours of daylight: the sun rises about 06:10 and sets about 18:21 local time in mid-September. Average cloud cover is 87%.

Is September a good time to visit Koh Samui?

It is a reasonable choice: hot days but expect showers. The best months for Koh Samui on our scoring are February, March, January; months averaging 25°C or more: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec.

Hot days, with little cooling after dark

The heat is consistent rather than extreme: every day reaches 25°C, while only 9% reach 30°C. That makes September suitable if you want warmth without routinely facing much higher temperatures, although humidity of 80% can make being outdoors feel sticky. Keep strenuous plans away from midday, wear loose, breathable clothing and leave room for shaded breaks. Nights average around 26°C, so evenings offer little relief from the daytime warmth; air conditioning is a sensible accommodation priority if you struggle to sleep when hot. Pack mainly light clothes rather than expecting the cooler evenings you might associate with a September holiday closer to home.

Warm water, but limited sunshine

The main catch is the combination of frequent rain and little bright sunshine. With about 1.6 hours of sunshine across 12 hours of daylight, this is not a strong choice for a holiday built around sunbathing. Rain on about 23 days means you should expect wet weather to feature regularly, though that does not mean every rainy day is a complete washout. Keep outdoor plans movable and pack a light waterproof that will not feel too heavy in the heat. The sea averages 29.1°C, so water temperature is comfortably suited to swimming. Treat that as a reason to pack swimwear, not a guarantee that conditions will always be suitable for going in.