The coldest temperature ever recorded in Thailand was -1.4c (29f) in Sakon Nakhon, a city in the northeast of the country. This record was set in 1974.
The coldest temperature ever recorded in Bangkok was 50°F (10°C) in 1955. In January 1955, the temperature in Bangkok was 49.8°F (9.9°C).
However, if you want to see frost or even snow, Doi Inthanon in Chiang Mai is the place to go.
You will need winter clothing.
Temperatures in Doi Inthanon can routinely drop before freezing and after being in the heat all year, it feels much, much colder