September is ‘low season’ across much of Thailand as large swathes of the country are under the influence of the Southeast monsoon.
Although rainfall is higher between May and October, rainfall is usually concentrated in short sharp bursts, often in the form of a late afternoon downpour, and many visitors to Thailand during these months are pleasantly surprised to experience long sunny days with little rain.
This can be an excellent time of year for a beach holiday in Thailand as many resorts on the West Coast have half price or two for one offers.
Koh Samui can see a little bit more rain in September than in previous months but September can still be an excellent month to visit. Expect an afternoon rain shower or two however, especially later in the month.
Bangkok and Chiang Mai follow the same season as the West Coast of Thailand.