The Hoops boss has previously worked with the Dynamo Kiev midfielder at both Portsmouth and Tottenham Hotspur, from where he has recruited two of his other former players today.
Redknapp told QPR's official website: "Niko's a crowd pleaser – he gets people off their seats. He's a top player and someone who will add great quality to the squad.
"He can play in a variety of positions – off the frontman, out wide on either side – and he's always a threat. He gets at players and makes things happen.
"He's someone I obviously know a lot about and he'll be a fantastic addition for us."
Twenty-nine-year-old Krancjar, who left the Lillywhites for Kiev in 2012, follows the Spurs pair in joining Championship outfit Rangers on a season-long loan deal.