December has the most reliable weather of the year and the highest prices, especially 20 December to 5 January. Expect dry, sunny days at 28–32°C on the coast and cool, clear nights in the north.
The short version
- The best weather of the year on both coasts.
- The most expensive two weeks are 20 December to 5 January — book by September.
- The north gets cold: 10–15°C at night in Chiang Mai and Mae Hong Son.
- New Year's Eve in Bangkok or on an island is worth planning around.
Weather
Dry, sunny and 25–31°C almost everywhere. The Andaman is at its calmest; the Gulf can still see rain in the first half, so Samui is better after mid-month.
The north gets genuinely cool at night — 15°C in Chiang Mai is normal, and hill towns drop below 10°C.
The Christmas squeeze
From 20 December to 5 January prices roughly double and the good hotels sell out. If you want that window, book by September; if you can move, the first two weeks of December are excellent value.
What to do
Beach first, cities second — Bangkok is pleasant when the heat breaks. Book boats early; the good operators are full every day between Christmas and New Year.
Where to go in December
December is the one month where you can go almost anywhere and expect good weather, which makes it a choice about atmosphere rather than risk. The Andaman for beach and boats, the north for cool mornings and clear mountains, Bangkok for the city at its most comfortable.
The north is the surprise for first-timers: Chiang Mai in December is 12–15°C at dawn, with mist in the valleys and log fires in the hill-tribe villages. Bring a jacket, and book the mountain hotels early since Bangkok escapes north for the long weekend.
If you are in Thailand for New Year, the choice is Bangkok (riverside dinners and fireworks) or an island (Koh Samui and Phuket both run large parties). Both need booking by September, and both charge a premium for the 31st that is not always reflected in what you get.
