British Olympic gold medallist Andrew "Bart" Simpson died when the Swedish catamaran training for the America's Cup capsized in San Francisco Bay.
Artemis Racing said Simpson, their strategist, died after the boat's platform trapped him underwater for about 10 minutes around 1pm local time.
It is being reported that New Zealander Craig Monk was injured in the capsize, suffering a cut to his neck. His injuries are considered serious, but not life-threatening.
Artemis said doctors "afloat" with the team and on shore couldn't revive Simpson after he was freed from the wreckage.
"The entire Artemis team is devastated by what happened," CEO Paul Canyard said in a statement on the team's website. "Our heartfelt condolences are with Andrew's wife and family."
The 36-year-old Simpson won the Olympic Star class in 2008 and the silver medal at last year's London Games. He also won the world title in 2010.
Simpson and another sailor were injured when the catamaran capsized near Treasure Island. Both were brought to shore and taken to the St. Francis Yacht Club, where paramedics continued to perform CPR on Simpson.
The other sailor suffered minor injuries, and the rest of the 11-man crew was accounted for and taken back to their dock in Alameda in a boat operated by Oracle Racing, the America's Cup holder.
The boat that capsized is a specially built 72-foot (22-meter) catamaran that can reach speeds of 45 mph (72 kph).
Coast Guard Lt. Jeannie Crump said it did not know the extent of the damage to the boat, but she added a commercial salvage boat was on scene to tow the catamaran to Clipper Cove.
She added that Coast Guard officials weren't sure what caused the catamaran to capsize.
Team New Zealand have released a statement saying they are shocked by Simpson's death and have extended their condolences to Simpson's family, friends and to the Swedish team.
Artemis Racing, representing the Royal Swedish Yacht Club, is the challenger of record for the cup.
The Louis Vuitton Cup for challengers starts on July 4, with the winner facing Oracle in the 34th America's Cup from Sept. 7.
This is the second time a sailor has died during training for the America's Cup.
In 1999, Martin Wizner of the Spanish Challenge died almost instantly when he was hit in the head by a broken piece of equipment.
Artemis has had its share of upheaval in the buildup to the 34th America's Cup. Late last year, American skipper Terry Huthinson was released. He was replaced by Nathan Outteridge of Australia, who won a gold medal at the London Olympics.
Artemis has had technical problems, as well. Last autumn, Artemis said the front beam of its AC72 catamaran was damaged during structural tests, delaying the boat's christening. A year ago, Artemis' AC72 wing sail sustained serious damage while it was being tested on a modified trimaran in Valencia, Spain.