Was Stieg Larsson Jewish?

No, the best-selling author Stieg Larsson was not a Jew.

Larsson's books are the "immensely successful Millennium trilogy" of which  the first two are, The Girl Who Played With Fire and The Girl With the Dragon Tattoo. The third book, The Girl Who Kicked the Hornet's Nest was reviewed in the Times Book Review.

The Times magazine's feature article in May 2010 on "The Afterlife of Stieg Larsson" explained the phenomenon of his books and the controversy that ensued over them.

...The novels come from Sweden, of all places, where the first one was published in 2005 and the next two over the following couple of years. They’re crime thrillers about a journalist named Mikael Blomkvist, who works for the magazine Millennium, and his sometime partner Lisbeth Salander, a startling and strangely appealing character who is a tattooed and pierced, bisexual computer hacker. Together this improbable pair solve mysteries involving spectacularly corrupt businessmen and politicians, sex traffickers, bent cops, spineless journalists, biker gangs and meth heads...

One character in The Girl Who Played With Fire is Jewish. Inspector Bublanski, of Polish Jewish descent, is described as wearing a Yarmulka around on the Jewish holidays, eating vegetarian burgers and yet working on the Sabbath.

The Timesonline reported details of Inspector Bublanski's Stockholm geography:

Officer Bublanski lives in the fashionable SoFo area, on Katarina Bangata. He is Jewish and worships at the synagogue on St Paulsgatan 13, a former cinema. But when he needs to think he heads instead to the Katolska Domkyrkan, the Roman Catholic Church, near his house on Folkungagatan 46.