The 27-year-old came through the ranks at Fratton Park and made his debut in a Premier League draw against Bolton in May 2005 before moving to Sweden two years later.
He told Pompey's website: "It was a long time ago that I was last here and it feels great to be back - it's nice to come home.
"The club's position has changed a lot, but there are still a few familiar faces around the place.
"I left here when I was 18 so it's been a while, but I'm looking forward to playing in England again for a massive club.
"I made my Pompey debut at Fratton Park and it'll be nice to get back out on the pitch again.
"I'll work hard for the team and try to help the club win some games - hopefully I'll provide a few goals as well.
"It's going to be big challenge - everyone understands that. No-one said it's going to be easy, but I'm looking forward to what lies ahead."
Keene is expected to go straight into the squad to face Notts County on Saturday.
Meanwhile, back-up goalkeeper Phil Smith has agreed a new one-month deal.