The decision has been taken in view of negative impact on natural resources due to heavy footfall at the island that is too beautiful for its own good.
The island of Koh Tachai, off the Phang Nga province, is part of the Similan National Park and is known for its beaches and coral reefs.
Most of the marine national parks in Thailand remain closed from mid-May to mid-October, for monsoon season, but Tachai will not reopen, the Bangkok Post reported.
The park is popular with tourists and divers - who will still have access to a few dive sites in the area.
"We have to close it to allow the rehabilitation of the environment both on the island and in the sea without being disturbed by tourism activities before the damage is beyond repair," Tunya Netithammakul, director general of the Department of National Parks, Wildlife and Plants Conservation, told the Post.
The island can accommodate only 70 people, but sometimes the number of tourists was well over 1,000, along with food stalls and tour boats. This has resulted into damage to the resources. Thailand draws tens of millions of tourists a year, many of them to its beautiful beaches and islands.