The law creates a Northeast Ecological Corridor along 2,900 acres of the island's northeast coast, the Puerto Rico news website Vocero reported.
Enactment of the law ends a 15-year battle between environmental activists and developers who wanted to build hotels, golf courses and luxury homes in the area.
"Today this important, highly ecologically valuable resource is being protected forever ... History is being made," Gov. Alejandro Garcia Padilla said as he signed the law Saturday.
In addition to being a nesting site for the highly endangered leatherback turtles, the newly protected area is home to more than 800 different types of flora and fauna.
Female leatherback turtles lay eggs in more than 400 nests they carve out on the protected beaches, where waves help push the 2,000-pound animals out of the water.