Thailand's busiest period is generally November to February, when cooler, drier weather attracts international visitors. December and January are particularly busy, with Australian Christmas and New Year holidays adding demand. Phuket, Krabi, Bangkok and Chiang Mai can all be crowded, while Songkran, the Thai New Year festival in mid-April, creates a separate domestic-travel peak. May to October is quieter in many destinations, especially during the wettest months, although some Gulf islands have different seasonal patterns and can remain popular when the Andaman coast is rainy.

Thailand's rainy season generally runs from May to October, bringing monsoon rain, flash flooding, river flooding and landslides. Tropical storms can also cause coastal disruption and transport delays, especially later in the wet season. Earthquakes and tsunamis are possible but are not predictable seasonal events. December to February is generally less exposed to heavy-rain disruption across much of the country, although southern Thailand has different rainfall patterns on its two coasts. Local conditions and warnings matter more than the calendar alone.

Thailand is hot year-round and has broad cool-dry, hot and rainy seasons. November to February is generally cooler and drier across much of the country, making it comfortable for sightseeing. March to May is the hottest period, with high temperatures and humidity. The southwest monsoon brings rain to many regions from roughly May to October. Regional differences are important: Phuket and Krabi are often wettest during this period, while Koh Samui and nearby Gulf islands typically receive their heaviest rain later, often from October to December.