The ex-West Ham defender has signed a four-year deal at Fratton Park after agreeing the terms of a transfer thought to be worth around £4million to the London club.
Johnson told Pompey's website: "It's a fantastic move for me.
"Portsmouth's a fantastic club with a lot of nice people. I learnt that when I was here last season.
"So it was an easy decision to make. It feels like I've never been away.
"The fact that I played here last season was a massive factor in my decision - I knew what the club was like.
"Harry Redknapp was a massive factor, too.
"I had to think about the move a bit, it's nice to have interest from other clubs.
"But deep down I knew where I wanted to go, especially after the success we had last season, when I enjoyed it here."