Crewman John Fisher, who now lives in Adelaide, Australia, fell from Team Sun Hung Kai’s Scallywag yacht 1,400 nautical miles west of Cape Horn at 1:42pm GMT yesterday.
Volvo Ocean Race organisers said a search was underway for the 47-year-old, but acknowledged the severity of the forecast and lack of light meant hopes of a rescue were slim.
They said the Scallywag team aboard the 65ft yacht carried out an exhaustive search for Mr Fisher, but were forced to turn downwind and head towards the nearest safe landfall on the South American coast.
“The weather in the area is forecast to deteriorate significantly in the coming hours,” they said in a statement.
“SHK/Scallywag has thus made the difficult decision to turn downwind and head towards the South American coast, the nearest safe landfall, approximately 1,200 nautical miles away.”
The wind in the area at the time was a strong 35-knot westerly and the water temperature was 9C.
The Maritime Rescue Coordination Centre requested one ship divert to the scene and is continuing to contact other ships that may be able to assist.
Mr Fisher was taking part in his first Volvo Ocean Race, a 45,000 nautical mile race around the world. The teams were on leg seven from Auckland, New Zealand, to Iajai, Brazil, when Mr Fisher went missing.
He was on watch and wearing appropriate survival gear when he went overboard, the organisers said.