The rainy season in Thailand varies from region to region and is largely dominated by the monsoon. Very broadly speaking, Thailand’s rainy season can be classified as May/June to October. For most of Thailand, the wettest months are usually August-October. However, the Gulf Coast of the southern peninsula (e.g. Ko Samui, Ko Pha Ngan) is affected by the south-west monsoon which can lead to heavy rainfall in November and the beginning of December.
Don’t be put off by the thought of visiting Thailand during the rainy season. Yes, the rain can be torrential, but the weather is still very warm and the downpours are often short and sharp. Bargains can be had with accommodation and the tourist areas are less crowded.
Statistics from Thai Meteorological Department