Twenty-six of the 77 Thai provinces - comprising three-quarters of the country - have been hit by floods this rainy season, affecting 2.6 million people, AAP reports.
There is some concern that the floodwater may flow into the capital Bangkok, as flooding continued to worsen, Al Jazeera reports.
The disaster, which has also affected part of the ancient city of Ayutthaya, has partly been blamed on bad management of the country's large dams, AFP reports.
Officials have been forced to release water from its two largest dams, Bhumibol and Sirikit.
"We kept too much water in the dams early in the rainy season, and now at the end of the season, they have to release a large amount of water at the same time, which has caused floods," said Smith Dharmasaroja, head of the Natural Disaster Warning Foundation.
On Wednesday, 400 families were forced to evacuate Bang Phrahan district in Ayutthaya province, 90km north of Bangkok, as the Chao Phraya River overflowed its banks, AAP reports.
Smith blamed the rains on a lengthy La Nina cycle, a weather phenomenon marked by heavy precipitation that typically follows an El Nino period of drought.
"This La Nina started early and was supposed to end six months ago, but it's continued up to now," Smith said. "It's quite unusual."
Northeastern Thailand can expect more rain this week from a low-pressure front left over from Typhoon Nalgae, National Disaster Prevention Centre Director Somsak Khaosuwai said on the department's website.
Nalgae hit the Philippines over the weekend and moved on to Vietnam Wednesday.