Puerto Rico has introduced a new law protecting a swathe of the island's coast that has become a major nesting site for the world's largest turtle, the leatherback.
The Northeast Ecological Corridor comprises 14 sq km (5.4 sq miles) of the island's coast.
The law ends a 15-year battle which pitted developers against green activists and several celebrities.
Leatherback turtles are a highly endangered species.
"Today this important, highly ecologically valuable resource is being protected forever... History is being made," said Governor Alejandro Garcia Padilla, according to the island's Vocero news site.
Developers had been looking to build hotels, golf courses and luxury homes in the area, arguing that this would boost the local economy and create jobs.
But the area - boasting lush vegetation and pristine beaches - is now likely to become a centre for eco-tourism.
As well as being a nesting site for the leatherback turtle, the area is home to more than 860 different types of flora and fauna.
It also contains a bioluminescent bay, featuring micro-organisms which glow in the dark.
Leatherback turtles weigh around 600kg (95st) and their shells can be up to two metres (6ft 7in) long.
The shell is flexible and covered in a black leathery skin - hence the name leatherback.
Last August thousands of leatherback eggs and hatchlings were crushed by bulldozers moving waterlogged sand from key nesting areas.