Some villages were battling water levels two metres high and soldiers had been sent to evacuate people to higher ground, especially in areas most susceptible to flash floods.
Al Jazeera's Selina Downes reporting from the village of Ban Mi in central Thailand said the flooding was the worst seen in three decades.
Emergency crews have begun delivering relief supplies, including food, water and medicine, to affected areas, Thai officials said.
Emergency sanitation equipment and special water purification tanks are also being brought in.
Thailand's meteorological department has warned that more heavy downpours and flash flooding could be expected soon.