John Fisher, 47, had been competing with Hong Kong entry Team Sun Hung Kai/Scallywag on leg seven of the race from Auckland to Itajai in Brazil and was reported missing in extremely rough seas on Monday.
The 65-foot yacht's location was some 1,400 nautical miles west of Cape Horn, and 1,200 miles from the nearest safe landfall in South America. The nearest competitors were 200 miles away downwind.
Fisher, a Sydney-Hobart veteran who was born in Southampton but based in Adelaide, had been on watch and was wearing appropriate survival gear.
"Given the cold water temperature and the extreme sea state, along with the time that has now passed since he went overboard, we must now presume that John has been lost at sea," race president Richard Brisius said in a statement.
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"The crew is, of course, emotionally and physically drained after what they have just experienced. Our sole focus now is to provide all the support and assistance that we can to the team," he added.
Team Sun Hung Kai/Scallywag had carried out a search for several hours before heading in a north-easterly direction with the weather deteriorating and conditions challenging.
A ship had been diverted to the scene but remained more than a day away.
The 2017-18 Volvo Ocean Race started from Alicante, Spain, on Oct. 22 and covers 45,000 nautical miles (83,000 km) around the world, featuring a total of 12 host ports. It ends in The Hague, Netherlands, at the end of June.
There has already been one fatal accident involving the race, with Vestas 11th Hour Racing withdrawing from the fourth leg in January after a collision with a fishing boat on the approach to Hong Kong.
One member of the fishing vessel's crew died after being taken to hospital by helicopter.
(Reporting by Alan Baldwin; Editing by John O'Brien)
