The 25-year-old Karlsson was given a one-year contract on Wednesday. He posted a 2.45 goals-against average and a .921 save percentage in 32 games for Malmo of Allsvenskan in Sweden last season. He also had a 2.49 goals-against average in seven games with Sodertalje of the Swedish Elite League.
Sharks general manager Doug Wilson also announced a two-year deal with Greiss, who was a restricted free agent.
Greiss, who played three games with the Sharks in 2007-08, spent last season with Worcester of the AHL.