In 2010, Expressen published an article in 2010 in which a journalist had purchased illegal weapons in Malmö. The decision to buy a gun followed a spate of fatal shootings in the southern city, most of which were suspected to have links to organized crime networks.
The reporter managed to buy a gun within a few hours, which the newspaper chose to write about before its staff handed the weapon over to the police.
He and his bosses were subsequently charged for violating Sweden's weapons laws.
"The Court of Appeals does use several phrases where it appears to understand why we used the methods we did," editor-in-chief Thomas Mattsson said in a taped statement in reaction to Tuesday's verdict.
"Yet they still found our way of working to be illegal."
He said the newspaper would now consider whether to appeal the case to the Supreme Court (Högsta domstolen), understanding there is no guarantee that high court would review the case.
According to the verdict issued on Tuesday by the Svea Court of Appeal, the reporter, Diamant Salihu, is guilty of gun crimes, while Mattsson was convicted of conspiracy to commit gun crime, alongside senior news editor Andreas Johansson who was convicted as an accomplice.
They have been given a suspended sentence and are expected to pay fines.