Thailand has banned sunscreens containing chemicals that damage coral from all of its marine national parks.
Concerns are growing that lotions tourists use for sun protection are harming slow-growing corals.
Anyone flouting the ban can be fined up to 100,000 baht (£2,100).
However, officials have not said how they plan to enforce the new rule.
The banned lotions are those containing oxybenzone, octinoxate, 4-methylbenzylidene camphor or butylparaben.
Similar bans have been introduced by the Pacific island of Palau and the US state of Hawaii.
This is the latest attempt by the Thai government to protect its coral from the tourism industry.