The publishing house calls it "the fourth book in the series", and it will be written by Swedish journalist David Lagercrantz, whose recent work includes "I am Zlatan", the autobiography of Sweden's biggest football star.
The new book picks up where Larsson, who died in 2004 of a heart attack, left off and is scheduled to be released in August 2015.
Aftonbladet reports that Norstedt chose Lagercrantz because of his success with Zlatan's book. "That is correct. We have been thinking back and forth if we should do this. Eventually, the thought was that David could be the author we were looking for," group publisher Eva Gedin told the tabloid.
The new book will not be based on any notes left behind by the late author, and instead will be a stand-alone sequel to the series. It will be released on the tenth anniversary of publication of the first book, "Men who hate women/The girl with the dragon tattoo", next year.
Lagercrantz told Aftonbladet that he was drawn to the best-selling series by its memorable characters and will impart his own take on the story while staying true to Larsson's legacy.
"I am creating my own intrigue, but he has left strings of ideas for me to follow. I will carry on his legacy, I will keep his characters, but I also have to give something of my own," he said. "The characters will continue but also evolve."
But Stieg Larsson's common-law wife, Eva Gabrielsson, tells Aftonbladet she thinks the plans are "tasteless". She had not been informed about the plans, and adds: "I think it is just a little greedy. It is already a billion dollar industry. It's rather tasteless to try to make more money," she told the tabloid.
The trilogy, telling the story of hacker Lisbeth Salander and journalist Mikael Blomkvist, has sold millions world-wide and has been filmed both in Swedish and English. Larsson died before the first book was even published.