Perch joins the Magpies for an undisclosed fee having passed a medical and signed a four-year deal.
According to Chris Hughton, the 24-year-old can play anywhere across the back four and in midfield.
Chris Hughton: "James is a player I've admired for some time and it's great we've been able to secure his services.
"I'm certain he'll prove to be a great addition to our squad."
Forest's chief executive Mark Arthur: "James has been a marvellous servant for Nottingham Forest.
"He made it clear that the prospect of a move into the Premier League with Newcastle appealed to him and we couldn't stand in his way.
"I would like to thank him for the service he has given us and wish him all the very best with Newcastle."