Redknapp, who was fired by fellow London club Tottenham Hotspur in June, has agreed a two-and-a-half-year contract, the club announced, although the manager himself later said that the deal had yet to be signed because of the speed of events.
Hughes's assistants Mark Bowen and Eddie Niedzwiecki were put in temporary charge of the team for Saturday's daunting trip to face Manchester United in the league and Redknapp would watch from the stands he told Sky Sports News.
QPR, who narrowly avoided relegation last season, have not won any of their 12 league matches this season under Welshman Hughes, who lost his job after 10 months at Loftus Road.
"I'm delighted to be back in the managerial hot-seat at QPR," Redknapp, 65, told the club website.
"There's no doubt there's a lot of ability in the squad but for one reason or another things haven't gone as well as everyone expected since the start of the season.
"I can't wait to get to work and get the results necessary to kickstart our season."
Regarded as one of the best managers in the English game, Redknapp had been close to accepting the job of Ukraine coach this week, but said on Friday that he would be the happiest man if he could save Rangers from relegation.
"It was a really interesting job," Redknapp said of the Ukraine post in a telephone interview on the BBC on Saturday. "I had met the people and to be honest it was almost a done deal.
"Then I got the phone call about QPR and I just couldn't resist the Premier League.
"I haven't even signed my contract yet," Redknapp added. "It has been a bit hectic since yesterday."
QPR chairman Tony Fernandes said Redknapp had been the club's top target for the job.
"His man-management skills are second to none and we are confident he is the right man to lead us away from the bottom of the table," he told the club website.
QPR are four points behind second-bottom Southampton in the league and Redknapp said his first task would be to motivate the players.
"At the end of the day the players obviously haven't performed," he told the BBC. "We have to get their confidence up, we have to get them working and doing what they are paid to do and that is winning some football matches."
The club said Redknapp's back-room team would be announced later.
Redknapp was travelling to Manchester to watch the United game and said he was calling the players in for training on Sunday and would be in charge for the first time when QPR play Sunderland at Loftus Road on Tuesday.
Redknapp, who spent the bulk of his playing career with West Ham has now taken charge of his third London club after also managing West Ham and Spurs as well as stints at Bournemouth, Portsmouth and Southampton.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.