"That Which Does Not Kill" was completed in November by David Lagercrantz, known for co-authoring Swedish football star Zlatan Ibrahimovic's autobiography.
Larsson himself died of a heart attack in 2004 aged 50. He left the outline of the fourth novel.
The 500-page sequel takes up the story of tattooed computer hacker Lisbeth Salander and journalist Mikael Blomkvist, whom she helps solve a murder mystery with a misogynist Nazi twist in 'The Girl with the Dragon Tattoo', the first book in the series.
"One night professor Frans Balder, a leading authority within AI (artificial intelligence) research, calls up Blomkvist," Norstedts publishing house wrote on its website, setting the scene for the new book.
"Balder says he has world shattering information on US intelligence services… had contact with a female super hacker, who bears a certain resemblance to a person Blomkvist knows well."
Until now the publishers had remained tight-lipped about the plot, only saying that it would continue the story of the trilogy's heroine, which has sold more than 75 million copies worldwide and become a popular film franchise, with both English and Swedish movie versions of the stories.
The new book's production has been shrouded in secrecy with the author, editors and translators reportedly working on computers disconnected from the internet to avoid leaks.
"If you write about Lisbeth Salander its easy to get a little paranoid. You realise what opportunities there are to hack computers," Lagercrantz told the Dagens Nyheter daily.
"It was a little awkward. Every time I needed to google I had to do it on another computer. And we used code words — like our own little spy world."
At the time of his death, Larsson reportedly had plans to write at least another seven novels.
His late partner Eva Gabrielsson recently told news agency AFP that the journalist, who co-founded the anti-racism magazine Expo in Sweden, "would be furious" if he knew someone else was writing his story.
Gabrielsson, who lost a bitter battle with the late author's family to manage his work, added that Lagercrantz was "a totally idiotic choice" as he lacked Larsson's left-wing activist background.
Norstedts have compared the planned release on August 27th of the book in at least 35 countries to the global splash made by Dan Brown's sequels to the 2003 'Da Vinci Code'.