More than 245 marine scientists from 35 countries made the call for the equivalent of national parks to be established at sea to mark World Oceans Day today.
The experts signed a statement issued by Global Ocean Legacy, a project of the Pew Environment Group, urging governments to take bolder action in establishing large protected areas in the seas.
Conservationists warn that overfishing, pollution and climate change are damaging the world's oceans, with impacts on livelihoods and food security for millions of people.
But establishing large-scale marine reserves can protect fish populations, providing them with a refuge to breed and spawn in the face of pressure from fisheries.
And reserves can promote healthy ecosystems which are more resilient to damage from pollution, climate change and other human impacts.
Professor Callum Roberts, from the University of York, said: "Extensive marine reserves are essential if we are to conserve the ocean's resources.
"If we wish to preserve the few unspoiled areas that we have left, then we must protect them now."
Alistair Gammell, of the Pew Environment Group, pointed to the decision earlier this year by the UK government to designate the Chagos Islands and surrounding waters, south of the Maldives in the Indian Ocean, which cover some 210,000 square miles, as the world's largest no-take marine reserve.