The 33-year old has returned to Bramall Lane after departing for Stoke City in January 2009 and will stay with the Blades until mid-January.
"I'm delighted to be back. Sheffield United is a club that is very close to my heart," he told the club's official website.
"I was at the [Carlisle] game on Saturday and the fans were singing my name, which was nice and this was always one of the clubs I wanted to return to.
"I've signed a deal until January, which is good for me, but more importantly the club, given I haven't played much at all in the last 18 months to two years."