The Department urged residents of Northern, North Central, North Western and Eastern provinces to exercise extreme caution when outside, reports Xinhua news agency.
Those living in other parts of the country have been asked to take precautious measures.
The Department warned that staying outdoors for extended time periods can cause many health complications, including heat strokes.
Sri Lankans have been asked to stay hydrated, remain in the shade for as much as possible, avoid strenuous work, wear light-coloured clothes, and not to leave children alone in vehicles.