The government's move aims to speed up relief operations, as the floods have left at least 281 people dead and damaged millions of homes and livelihoods in more than two months.
"The government has announced that all provinces affected by the floods are critical disaster areas, allowing governors to exercise more authority to issue materials and manage budgets," deputy premier Yongyuth Wichaidit said.
Currently 26 out of 77 provinces are affected, while the capital Bangkok is bracing for a large amount of run-off water to reach the city in mid-October, when high tides will make it harder for the flood waters to flow out to sea.
Officials have bolstered flood defences at the main airport and other areas to shield the city of 12 million people, a number of whom have been stocking up on sandbags, non-perishable food and other essential items.
Areas just north of the capital have already seen water up to several metres deep. Prime Minister Yingluck Shinawatra said that those living outside the beefed-up defences needed to prepare themselves for flooding.
"If inner Bangkok is flooded, it will only flood a little, but we should be concerned for those who live outside the barriers," she told the press on a visit to a flood relief operations centre at Bangkok airport.
The premier met King Bhumibol Adulyadej on Wednesday to discuss the crisis.
Japan's biggest automaker Toyota said that production at its three Thailand plants would be halted at least until Saturday.
While floods have not directly impacted its Samrong, Gateway and Ban Pho operations, Toyota said they have caused disruption to parts supply.
Operations at the three plants have been halted since Monday and the company will decide Saturday on whether to resume production.
The flooding has also stuck countries neighbouring Thailand, including Cambodia, where more than 200 people have died. In Vietnam, the toll has risen to 34 and officials say most of the dead are children.