The Blades can confirm an agreement has been reached with Blackpool for a 28-day loan swap.
Bob Harris, primarily a left-back, arrives at Bramall Lane, with Tony McMahon moving to Bloomfield Road in exchange.
On the deal, boss Nigel Clough said: "Bob is someone we have chased for a while and looked seriously at trying to get him in the Championship last season.
"We have had to sacrifice Tony McMahon to get the deal done but, fortunately, we have managed to bring in John Brayford, which gives us further defensive options. We were not looking to move Tony on but with his contract up in the summer this gives him a chance to impress at Championship level."
Glasgow-born Harris has featured three times for the Tangerines this season, the last of which came at Millwall in September.
Prior to moving to Bloomfield Road in the summer of 2011, Harris played north of the border with Clyde and Queen of the South, where he made over 100 appearances.
Last season, the 26-year-old enjoyed a successful stint in League Two with Rotherham United, making five starts during a month's loan at the New York Stadium.
Harris will be available for selection on Sunday when United host Fulham in the FA Cup. He is the third capture of the day as www.sufc.co.uk has also confirmed the signatures of Stefan Scougall and John Brayford.