The court also ordered the four -- Fredrik Neij, Gottfrid Svartholm, Peter Sunde and Carl Lundstroem -- to pay damages of 30 million kronor (2.72 million euros, 3.56 million dollars).
"The Stockholm district court has today convicted the four people charged with promoting other people's infringement of copyright laws," it said in a statement.
Representatives of the movie, music and video games industry had sought some 117 million kronor (10.6 million euros, 13.9 million dollars) in damages and interest for losses incurred from tens of millions of illegal downloads facilitated by the site.
Founded in 2003, The Pirate Bay makes it possible to skirt copyright fees and share music, film and computer game files using bit torrent technology, or peer-to-peer links offered on the site.
None of the material can thus be found on The Pirate Bay server itself.
The four have denied any wrongdoing. In the days prior to the verdict they indicated they would appeal it if they were convicted.
The Pirate Bay claims to have some 22 million users worldwide.