Gross domestic product declined by 9% in the three months to December compared to a year earlier, the National Economic and Social Development Board (NESDB) said.
Compared to the previous three months, the economy contracted by 10.7%.
The floods killed more than 700 people and covered two-thirds of the country.
Analysts said despite the poor numbers, the future outlook was not as bleak.
“The fourth-quarter figures reflected a worse-than-expected impact from the floods. The overall picture going forward will pick up,” said Pimonwan Mahujchariyawong from Kasikorn Research Centre.
The NESDB raised its forecast for 2012, saying it expects a rise in both public and private spending.
Thailand’s Prime Minister Yingluck Shinawatra has said the country will spend 350 billion baht ($11bn; £7bn) on infrastructure after the floods.
The manufacturing index fell for the fourth month in a row and exports fell for a second month in December.
Honda, Toshiba and Fujitsu have all cut their profit forecasts because of disruption to production lines.
Thailand’s Ministry of Commerce said it was targeting a 15% increase in exports this year.