Mr Fisher - who is a British citizen but lives in Adelaide - was "wearing survival equipment when he went overboard", race organisers said in a statement.
"All of us here at the Volvo Ocean Race organisation send our heartfelt condolences out to John's family, his friends and his teammates and we will do everything in our power to support them in this very hard time".
"The MRCC has already requested a ship, almost 400 nautical miles away, divert to the scene", organisers said.
Teammates have been conducting a search and rescue operation but organisers say that because of the conditions - which were due to deteriorate throughout the day - turning other competitors in the race around to help is not an option.
He was taking part in the 45,000 nautical mile race around the world for the first time. In fact, the team is now in a challenging position - the weather is deteriorating and is forecast to be quite severe on Wednesday.
The incident was a "heart-breaking" tragedy, he said.
Mr Fisher is an experienced big boat sailor who is sailing his first Volvo Ocean Race, according to the team's website.
"All of us here at the Volvo Ocean Race organisation send our heartfelt condolences out to John's family, his friends and his teammates and we will do everything in our power to support them in this very hard time".More news: Trump denounces 'horrible' terror attack in France
"The crew did everything they could to recover John, leading an extensive search and rescue operation in stormy conditions", he said.
"Yesterday, just after 1300 UTC [GMT], Race Control for the Volvo Ocean Race were informed of a man overboard situation by Team Sun Hung Kai/Scallywag".
On leg four of the race in January, Scallywag rookie Alex Gough fell overboard into the Pacific Ocean and Volvo Ocean Race organizers shared video of the dramatic rescue, saying that Gough was pulled aboard in less than 15 minutes. He is one of our own, a long-standing member of the team.
Organisers described it as the "defining" leg of the race, through "some of the coldest, roughest ocean in the world".
The team has since been forced to turn downwind and head towards the nearest safe landfall, on the South American coast.
He purchased the 100-foot yacht Scallywag, previously known as Ragamuffin, in 2016, of which Australian sailor David Witt had been the skipper.
"The main thing is, we got him back on board".
In May 2006, Dutchman Hans Horrevoets died when he was washed overboard while sailing for ABN Amro in the Volvo Ocean Race.