He recently had a loan spell at Fratton Park cut short through injury but has now made a permanent move to Pompey.
Villa boss Graham Taylor told www.avfc.co.uk: “He goes, and he knows this, with my good wishes. Steve found himself in a position where he wasn’t getting regular first-team football.
“He was one of our high earners, there’s no doubt about that. He had to make a decision as to where he was going to get his first-team football from.
“He obviously wanted to play and so he’s moved on to Portsmouth.
"Good luck to him.”