Bangkok, Central and Eastern regions can expect to see the mercury drop 1-3C until Wednesday.
Temperatures in the Northeast will drop 2-4C from last week, the department forecast.
Wednesday will bring a further drop in temperatures of 2-4C in upper Thailand.
The lower South will experience heavy rain in some areas as the northeast monsoon persists. Moderate winds are forecast for coastal Andaman and the Gulf of Thailand, with waves of 1-2 metres and 2 metres, respectively.
From Wednesday, the temperature drop in upper Thailand including Bangkok will be accompanied by strong winds.
The upper Gulf will see waves of 2 metres, and higher in thunderstorms, while waves in the lower Gulf will be 2-3 metres, rising above 3 metres in thunderstorms.