The 40-year-old, capped 42 times by Finland, joined the Latics at the beginning of 2015-16 and has gone to play 32 games and keep 15 clean sheets.
Wigan are currently second in the table and on an 18-match unbeaten run.
"If my experience can help what we want to achieve then that is fantastic," said the veteran keeper.
"I've really enjoyed my season here so far, the team is playing well, the club have been very good to me and it's been great to play for the manager [Gary Caldwell] with the way he does things."
Latics manager Gary Caldwell added: "Jussi has been absolutely tremendous since breaking into the team.
"I've said on several occasions when asked about him that there's nothing I can teach him about goalkeeping, or how to recover and train."
Wigan Athletic goalkeeper Jussi Jaaskelainen has signed an extended deal at the League One club that will expire in the summer of 2017.