Hartlepool have sold defender Jack Baldwin to Peterborough in a £500,000 deal.
The highly-rated 20-year-old has signed a four-and-a-half-year-deal at London Road having made nearly 100 appearances for the Sky Bet League Two side.
Posh boss Darren Ferguson told the club's official website: 'It is a really good signing. He is a young centre-half and I think he is going to be a really top player. We have managed to get it done. Most of the Championship have been watching him and a Premier League club too but we really like him.
"He reminds me of Ryan Bennett, the way he passes the ball, he is tough enough and he knows we can improve him. It is a signing that excites me.
"It is an area of the pitch that we needed to strengthen. We have only got three recognised centre-backs and Nathaniel Knight-Percival has been doing very well at left-back and I feel I want to keep him there at the minute.
"He is going to be a really good player for us. He has played a lot of games, he has played in midfield too, but we will work with him as a centre-half. He has good ability on the ball, he is very elegant, reads the game well and he is the one we have been after for a while now. I am delighted to get it done."