About 600 houses were flooded when the Pa Sak River overflowed in the northern province of Phetchabun on Tuesday morning.
Officials said heavy rain in the area caused the river to overflow for the third time this year.
The water was about half a metre above its banks. The floodwater poured into villages next to the river. There was no flood embankment to protect them, and about 600 houses and shops were 50-60 centimetres deep in water.
Three local schools were closed on Tuesday.
