The veteran boss says he is quitting his post because he will undergo knee replacement surgery in the coming weeks.
The 67-year-old took charge at Loftus Road in November 2012 but informed Chairman Tony Fernandes that he was resigning on Tuesday morning.
And QPR have announced that Les Ferdinand and Chris Ramsey have been placed in temporary charge of the team until further notice.
Redknapp has been under pressure this season and leaves with QPR second-bottom of the Premier League, having not earned a single point away from home all season.
QPR were expected to try and strengthen their squad while the transfer window was open but their only signing was Mauro Zarate from West Ham, who has been on the field for just 14 minutes since his arrival on loan at the start of January.
Redknapp said: “I have had such a fantastic time at QPR.
“I would like to thank the Board, the players and all my staff, and especially the supporters, who have been absolutely fantastic to me since I arrived at the club for their tremendous support.
Fernandes said: “I would like to take this opportunity to thank Harry for everything he has done for QPR during his time in charge.