The defender, who joined Spurs from Lens in 2006, joins the highly rated young midfielder in reuniting with former boss Harry Redknapp in the Championship on deadline day.
The Cameroon international has fallen out of favor under Spurs boss Andre Villas-Boas and had yet to feature for the north Londoners this season.
Danny Rose, who returned from a successful loan spell at Sunderland last season, is now first choice at White Hart Lane, while Jan Vertonghen and Kyle Naughton were often preferred to Assou-Ekotto last term.
The 29-year-old joins the highly rated midfielder in droping down a division, with former Tottenham boss Harry Redknapp having lured them to Loftus Road on a season-long loan.
Assou-Ekotto, who enjoyed cult status at White Hart Lane, made 155 Premier League appearances for Tottenham, scoring four times and proving eight assists.
"I am delighted to come here and really excited," Carroll told QPR's official website. "Harry was a big factor for me because of how he likes the game to be played.
"I've seen what Kyle Walker and Andros Townsend got from their loans at QPR and I hope I can enjoy similar success."