The versatile midfielder has joined the Royals until January 25 and will go straight into the squad for Saturday's game against Leeds.
Leigertwood brings a wealth of experience to the Madejski Stadium and manager Brian McDermott is delighted with his capture.
"We're delighted to get Mikele in on loan," he told the club's official website.
"He is a player who has earned promotion to the Premier League before, via the play-offs with Crystal Palace, and who has worn the captain's armband at QPR.
"Mikele brings a wealth of experience through the door and I am surprised QPR have allowed us to sign a player with such ability and experience."