The Netherlands international midfielder moved to Loftus Road from Norwich City on Wednesday for a reported fee of £7.5m.
Fer, 24, has put pen to paper on a three-year deal with the R's.
"Leroy's arrival is a massive boost for us," Redknapp told the club's official website. "He's another excellent signing and I'm delighted we've been able to get him to QPR.
"He played for Holland in the World Cup this summer and has all the attributes you'd be looking for from a central midfielder."
QPR return to action on Sunday when they play Tottenham Hotspur at White Hart Lane in the Premier League.