The 38-year-old is the Magpies' longest-serving player and has made just under 200 appearances during his two decades at St James' Park, where he has spent most of his career as backup.
The veteran, originally signed from non-league Seaham Red Star, has fallen behind Tim Krul and Rob Elliot in the pecking order this season and has decided to seek a new challenge when his contract expires in the summer.
|Evens||William Hill offer Evens for both teams to score in Manchester City v Newcastle
"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 rollercoaster 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."
Signed by Kevin Keegan, Harper has played under 12 different full-time managers during his spell at the club as well as spending time on loan at Bradford City, Gateshead, Stockport County, Hartlepool United, Huddersfield Town and Brighton & Hove Albion.
He will now look to go into coaching, as well as continuing his playing career at a new club.