The Northern Ireland International has been out of favour at Bramall Lane since the arrivals of David Cotterill and Greg Halford, so he has took the opportunity to join struggling Chartlon Athletic.
The loan deal will last until the 3rd January 2009. The Blades hold a recall clause after 28 days.
United Gaffer Kevin Blackwell said: "It's a chance for Keith to get his league career going again, he's been unfortunate because we are very strong down the right with the likes of Cotterill, Halford, and Dyer, but he has been playing well for Northern Ireland."