The South Pacific island chain, a self-governing parliamentary democracy in free association with New Zealand, declared a 700-square-mile sanctuary contiguous with one created last week by neighboring French Polynesia, the BBC reported Thursday.
"We are proud as Cook Islanders to provide our entire exclusive economic zone ... as a shark sanctuary," Teina Bishop, Cook Islands minister of marine resources, said.
"We join our Pacific neighbors to protect this animal, which is very vital to the health of our oceans, and our culture."
Shark fishing and possession or sale of shark products is now banned in an area totaling 2.5 million square miles, nearly the size of Australia, officials said.
About a third of the world's ocean-going shark species have been placed on the Red List of Threatened Species of the International Union for Conservation of Nature.
The Cook Islands and French Polynesia have joined other island nations around the world with sanctuaries including Palau, the Maldives, Tokelau, Honduras and the Bahamas.
"This is hopeful news for the world's sharks and our efforts to protect them," said Jill Hepp, director of shark conservation for the Pew Environment Group, which aided the Cook Island's effort to create the sanctuary.
"We are thrilled to see the Cook Islands become part of this global movement during a time when so many shark populations are threatened."