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, 23, returns to the south coast after spending last season on loan with Pompey.
Copyright (c) PA Sport 2007, All Rights Reserved.
Do YOU want to write for GiveMeSport? Get started today by signing-up and submitting an article HERE: http://gms.to/writeforgms