The 26-year-old will be reunited with Blades manager Nigel Clough, who has previously managed him at Derby County and Burton Albion.
Clough has admitted that he is delighted with the capture of the full-back, who has made just two appearances for Cardiff this season.
"This is a major coup for us because, believe me, John could have easily moved to teams in the division above. But we have a fantastic professional relationship and after long discussions, we have convinced him this was an ideal place for him to ply his trade for the rest of the season," he told the club's official website.
"We know exactly what we are bringing in with John, he is a great professional. He was absolutely tremendous for us at both Burton Albion and Derby County and he certainly deserved his move to the Premiership last summer.
"We must also thank Cardiff City for helping make this loan happen, because without their assistance it wouldn't have been possible for us to get the deal over the line."
Brayford joined Cardiff from Derby last summer for an undisclosed fee.