More than 360,000 households, or about a million people, have been affected by the floods that have damaged homes and schools and affected rubber and palm oil production, the Department of Disaster Prevention and industry officials said.
Television pictures showed abandoned cars submerged in murky waters in Prachuap Khiri Khan province where a torrent washed out a bride on the main road linking Bangkok to the south, causing the 200 km tailback, media reported.
The railway link to the south, and Malaysia, beyond has been severed for days.
Thailand's rainy season usually ends in late November but this year heavy rain has fallen well into what should be the dry season.
Southern Thailand is a major rubber-producing area and the wet weather has hit production. Palm oil plantations have also been flooded, industry officials and farmers' groups said.
In Nakhon Si Thammarat, one of the worst-hit provinces, television footage showed villagers commuting by boat.
"It's like a big pond," said resident Pattama Narai.
Nakhon Si Thammarat has had 493 mm of rain in the past seven days, 426 mm more than the average for this time of year, according to Thomson Reuters data.
Flooding regularly occurs in the May-November rainy season.
In 2011, widespread flooding that began in the north and flowed down to Bangkok crippled industry, killed more than 900 people and slowed economic growth to just 0.1 percent that year. - AFP