The government on Wednesday banned sunscreens containing chemicals that damage coral reefs in marine national parks.
The kingdom's sandy beaches have long been popular destinations for millions of tourists, but concerns are growing that the lotions they use as protection from the tropical sun are harming delicate, slow-growing corals.
An order came into force on Wednesday banning lotions containing oxybenzone, octinoxate, 4-methylbenzylidene camphor or butylparaben from marine national parks.
The announcement said the science showed the chemicals "deteriorate coral reefs, destroy coral larvae, obstruct their reproductive system and cause coral reef bleaching".
Thailand follows the Pacific island of Palau and the US state of Hawaii which have already imposed similar bans.
Violators face a fine of up to 100,000 baht -- although officials have not said how they plan to enforce the ban.
The key tourism sector has been devastated by the pandemic as the government imposed tough entry restrictions as part of efforts to curb the virus.