The 38-year-old's current contract expires at the end of the season, and Harper will not be signing a new one with the club he joined 20 years ago when he signed from local non-league outfit Seaham Red Star.
Harper, currently number three at the club, has played nearly 200 times for Magpies, but he has mainly been back-up during his time at St James' Park.
He has also had a number of loan spells away from Newcastle, featuring for Bradford City, Gateshead, Stockport County, Hartlepool United, Huddersfield Town and Brighton & Hove Albion.
Harper will not be hanging up his gloves at the end of the season, but will carry on playing while also looking to move into coaching when he leaves St James'.
"It has been a privilege to be a professional at such a fantastic football club," he told the Evening Chronicle.
"If you had said to me as a raw kid from Seaham I'd be at the club for 20 years as a professional, I would have said you were mad.
"People describe the club as an emotional roller-coaster and it certainly has been. I've had some fantastic times here.
"There have been some difficult times as well, but I have enjoyed it all."