The reserve around the Chagos islands is the world's larges.
By Roger Harrabin
Marine Protected Areas are heading for a 10-fold rise within a decade.
A report to a UN meeting on biodiversity in Hyderabad reports that more than 8.3 million sq km - 2.3% of the global ocean area - is now protected.
The percentage is small but the rapid growth in recent times leads to hope that the world will hit its target of 10% protected by 2020.
This would have looked most unlikely prospect just a few years ago.
The aspiration was agreed by the Convention on Biological Diversity in 2004 with a target date of 2012. Progress was so slow at first that the target was slipped to 2020 - with some researchers forecasting it would not be reached until mid-century.
But recently there have been huge additions - like Marine Protected Areas (MPAs) in the UK-controlled Chagos archipelago and US-controlled uninhabited territories in the mid-Pacific.
The Cook Islands recently announced a 1.1 million sq km MPA - that is four times the area of the UK land mass. New Caledonia's is even bigger - 1.4 million sq km.
Australia has added a further 2.7 million sq km to its listing of the Great Barrier Reef. Now 28 countries have designated MPAs of more than 10%.
But these statistics may not be quite so impressive as they appear as most of them are far distant from people who would be likely to over-exploit them.
And a recent paper on the demise of the Barrier Reef demonstrates that declaring an area protected does not necessarily shield it from distant influences like over-nutrification.
Mark Spalding from the Nature Conservancy, lead author of the report, told BBC News: "This is great news in the sense that the prospect looked so hopeless until recently. We really should manage to meet the 10% target now.
"But we have to ask whether the targets in themselves are enough - or whether governments need to be smarter to ensure that they're protecting the very most important areas.
"I don't want to knock any of the MPAs but some appear to be easy wins, where you could stick a pin on a map and maybe send a patrol vessel. We need more than that."
Dr Spalding said it was vital now for nations to concentrate efforts on MPAs near heavily-populated coastlines where marine resources were most at risk.
The UK government has been accused of dragging its feet after postponing by a year the introduction of MPAs around the coast of England.