Over 80% of the coral at each diving site had been damaged, National Parks, Wildlife and Plant Conservation Department chief Sunan Arunnoparat said yesterday.
Bleaching, or a whitening of coral as it loses its natural pigment, is caused by a rise in sea temperatures which has been linked to global warming. The dive sites which have been closed are in seven marine national parks.
They are the Hat Chao Mai National Park in Trang, Mu Koh Petra and Tarutao national parks in Satun, Mu Koh Chumphon National Park in Chumphon, Hat Nopparat Thara-Mu Koh Phi Phi National Park in Krabi, and Mu Koh Surin and Mu Koh Similan national parks in Phangnga.
The coral bleaching, which has been growing more serious since April, is said to be the worst in 20 years.
Up to 90% of coral in the Gulf of Thailand and the Andaman Sea has been bleached.
The department chief said he could not say how long the dive sites would be closed but diving activities probably would be banned until the end of the monsoon season in October.
Marine national parks would be closed for six months during the monsoon season.
Mr Sunan said curbs would be imposed on tourist visits to some sites, and public awareness of marine life conservation would be promoted in other measures to deter bleaching.
A task force will monitor the situation and issue measures to speed up coral rehabilitation, he said.
Praput Khorpetch, vice-president of the Phangnga Tourism Association, said the association was willing to cooperate with government efforts to protect marine life.
"We don't want to see just a closure of national parks to rehabilitate coral.
In fact, we want to see the government and private tourism operators work together to find a long-term solution," Mr Praput said.
"The Marine and Coastal Resources Department should not blame just the tourism sector for this problem. We received the message and have made an effort to limit the number of tourists but we don't have the power to do that. The government should impose a strict law on this issue."
Suchart Sirankanokkul, president of the Thai Hotels Association Southern Charter, suggested the government set up a meeting with tourism operators in the affected areas to discuss solutions to the problem.