Lauren has made nearly 250 appearances in the six-and-a-half years at the Gunners.
The Cameroon player have signed a two-and-a-half year deal with Portsmouth.
Pompey boss Harry Redknapp told the club website: “Lauren is an outstanding player. He is a great athlete and professional, very strong and quick, too.
“I’m delighted he’s decided to join us over a whole host of clubs that were after him.”
Lauren missed the whole of 2006 due to a knee injury.
He said: “This is a great move for me.
“Portsmouth are having a great season and are pushing for a place in Europe and I aim to be playing my part in that challenge.
“I have been very impressed with the club’s plans for the future, both on and off the pitch, and this was why I chose to come to Fratton Park.
“It will also be good to once again to team up with former Arsenal team-mates Sol Campbell and Kanu.”