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."

Researchers in Bristol found 8% more patients died after 30 days if they were operated on between 24 and 36 hours after admission to hospital.
The delay is thought to have caused 670 excess deaths in four years.
Project leader Timothy Chesser said it was the "first time" the benefits of early surgery had been revealed.
Data was collected by a team at Southmead Hospital from the National Hip Fracture Database, the largest such list in the world.
The study focussed on 241,446 patients across England and Wales who were admitted to hospitals with hip fractures between January 2011 and December 2014, and the mortality rate for these patients 30 days after they were admitted.
Guidance issued by the National Institute for Health and Clinical Excellence in 2011 called for patients to be operated on either the same day, or the day after, hospital admission.
But the new report says that even earlier surgery can improve outcomes for elderly patients who are often frail, with multiple medical problems.
"We found 8% more patients died if they were operated on between 24 and 36 hours compared to those given surgery within 24 hours, and the risk increased to 20% for those receiving surgery after 48 hours," said Adrian Sayers, the lead author on the paper.
Timothy Chesser, the clinical lead of the research project, said early surgery was not advisable for every patient, but was beneficial in the majority of cases.
"We have shown for the first time that early surgery is much better for patients," he said.
"The caveat is some of these patients are very sick and would benefit from greater time to get better before surgical procedures."

A charity wants older people to help care for pet chickens in an attempt to tackle loneliness.
The idea, by the project Hen Power, has experienced significant success with the scheme in England.
What started with a couple of hens in Northumberland has expanded to more than 40 care and residential homes across England and Wales.
Similar schemes are also taking place in Canada and Australia.
Caring through arts is being backed by the Arts Council of Northern Ireland, which is promoting other forms of art among the elderly in care settings including dance, music, creative arts and circus skills.
But will bringing chickens into care homes ruffle some feathers?
Not according to 80-year-old widow Pat Cain who is originally from Merseyside.
She lives alone in sheltered accommodation as part of a wider care home and has been a hen-keeper for four years.
"First of all some shook their heads, they didn't care for it at all. But now it's part of our lives, it would be strange if we didn't step out of our bungalows and care for the hens.
"It's made everyone around me change their lives."
The project recently received £1m of Big Lottery funding and Hen Power says it is hatching a plan for the scheme to be introduced in Northern Ireland soon.
Wigan Athletic goalkeeper Jussi Jaaskelainen has signed an extended deal at the League One club that will expire in the summer of 2017.
[NEXT_CONCEPT]
Hundreds of lives could be saved if patients with hip fractures were operated on in under 24 hours, a new study reveals.
[NEXT_CONCEPT]
The number of 'hensioners' in Northern Ireland could soon be on the rise.