Britain's John Fisher -- who is based in Adelaide, Australia -- was sailing on the 70-foot Team Sun Hung Kai/Scallywag in the Auckland, New Zealand to Itajai, Brazil leg of the prestigious around-the-world race when he went overboard in 35-knot winds and fierce seas Monday.
Despite an "exhaustive search" in "extremely challenging weather conditions," his teammates were unable to recover the 47 year-old, who was on watch and wearing appropriate survival gear when the incident occurred 1,400 miles west of Cape Horn.
"This is heartbreaking for all of us," Richard Brisius, the president of the Volvo Ocean Race, said in a statement.
"As sailors and race organizers losing a crew member at sea is a tragedy we don't ever want to contemplate.
"We are devastated and our thoughts are with John's family, friends and teammates... we will do everything in our power to support them in this very difficult time."
He added: "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."
The Maritime Rescue Coordination Centre (MRCC) is assisting in the search efforts and has diverted a ship to the scene. The rest of the Volvo Ocean Race fleet was some 200 miles downwind, making it unfeasible to sail back upwind through large seas to join the search.
The remaining crew on Team Sun Hung Kai/Scallywag are uninjured and are sailing northeast in deteriorating weather towards the nearest port in South America.
"The crew is, of course, emotionally and physically drained after what they have just experienced," added Brisius.
"Our sole focus now is to provide all the support and assistance that we can to the team."
The last fatality in the race was in 2006 when Dutch sailor Hans Horrevoets died after falling from ABN Amro Two while racing from New York to Portsmouth.
In November 2017, Britain's Simon Speirs was swept off the yacht Great Britain while sailing from Cape Town to Fremantle, Australia in the Clipper Round the World Race for amateur crews. The 60-year-old was recovered within 36 minutes but failed to regain consciousness.