Jack Ross reacts after making Jack Baldwin his ninth Sunderland signing
Ross has now completed his centre-back recruitment drive as Baldwin prepares to battle with Alim Ozturk, Tom Flanagan and Glenn Loovens for a regular place in the side.
Baldwin is believed to have cost the Black Cats around Â£200,000.
Ross said: "Everybody knew we were looking to strengthen again in this area of the pitch and now we’re comfortable with where we’re at in terms of central defensive options.
“Jack is proven in this league and he’s at a good age with his best years ahead of him, and he has a hunger to improve and do well.
"He sees this as a big opportunity and has the core attributes we have been looking for.”
Peterborough United chairman Darragh MacAnthony told supporters on twitter that he would miss the influential player.
He said: "Course but I have to do right by player and manager on things like this. Jack needs a change for himself also so I’m backing that & our manager.
"He’s a superb young man & I will of course miss him. This is football."
Baldwin has put pen to paper on a two-year deal.