Torrential rain has lashed the country for the last two weeks, causing flash floods and mudslides in almost half its provinces, with families evacuated from their homes in boats or makeshift rafts.
Since August 29, 32 people have been killed in the deluge, said a statement from the disaster department on Saturday that also gave the number of people staying in emergency shelters.
Two weather events are behind the widespread floods, the department said -- Storm Podul and a tropical depression that formed over the South China Sea called Kajiki.
Local media reports from the worst-hit province of Ubon Ratchathani showed people wading through chest-deep water and rescuers in boats trying to steer buffalo to higher ground.
Flooding in the province, which borders Laos and Cambodia, has been exacerbated by rising water levels in the Moon and Chi rivers.
"It will take three weeks to drain the floodwater" from up to 90 percent of inundated households, said provincial governor Sarit Witoon.
"The water has slightly receded about four centimetres today and I think it will keep going down," he added.
But the situation is already "unlivable" for families in one-storey homes, said Pongsak Saiwan, local director of opposition party Future Forward.
Access to an entire district is currently cut off due to flood waters, which are about two metres (6.5 foot) deep in the main town, while three major bridges are "impassable", he said.
"The government has been very slow in responding to the situation since the floods started in the beginning of September," Pongsak told AFP.
Ubon Ratchathani's plight started trending on Twitter this week with the hashtag #SaveUbon.
Aerial shots of the flood-hit plains blanketed with muddy river water were widely shared, as well as photos of stray dogs being rescued by passing boats.
One Twitter user compared the flood response to how quickly the government had mobilised and saved 12 young boys and their football coach from a waterlogged cave last year -- an incident that catapulted Thailand to international attention.
"Only 13 lives stuck in the cave and it was still very high-profile, but this is hundreds of thousands of lives," tweeted Yosita8051. "It's not okay."
Thailand's junta leader-turned-premier Prayut Chan O-Cha tweeted on Saturday that he has told agencies to "expedite assistance" to those in the affected areas.