The 25-year-old winger joins Chris Hughton's side for an undisclosed fee from the Loftus Road club.
Routledge, who has scored seven times in 49 appearances for the Londoners before his move to the north-east, was part of the Hoops side which drew 1-1 at St James' Park earlier this season.
The Magpies are Routledge's eighth club, not including the 12 games he played for England under-21s.
WAYNE ROUTLEDGE - A CAREER IN PICTURES Crystal Palace (left) 2001-05; Tottenham Hotspur 2005-08
Portsmouth (loan deal) 2006; Fulham (loan deal) 2006-07
Aston Villa 2008-09 (far left); Cardiff City 2008-09 (loan deal)
.and then there was the matter of just over a year at QPR after signing in the 2009 January transfer window
He began his career at Crystal Palace where he made 110 League appearances before joining Tottenham in 2005.
He played just a handful of games there in three years, instead being farmed out on loan to Portsmouth and Fulham.
Two years ago Routledge joined Aston Villa for £1.25m but was again sent out on loan - this time Cardiff - before signing for QPR last year.
Gordon Strachan keen on taking QPR winger Routledge to MiddlesbroughRoutledge: Villa will see the best of me - and I'm honoured to be hereJol hopes Routledge can find right route at SpursDerby 0 QPR 2: Routledge and Leigertwood spoil Clough partyTOTTENHAM HOTSPUR FC