The 26-year-old made the switch to Premier League Cardiff in a reported £1.5 million deal from Derby County in the close-season.
However, he has failed to break into the first-team picture at the Cardiff City Stadium, with the Welsh outfit deciding a temporary deal would benefit all parties.
The move sees Brayford link up once again with United manager Nigel Clough, who he has previously worked under at Burton Albion and Derby.
Clough is thrilled to have landed one of his top January targets, Brayford having reportedly courted interest from several Championship clubs in the transfer window.
"This is a major coup for us because, believe me, John could have easily moved to teams in the division above," Clough told United's official website.
"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.
"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 could make his debut as early as Sunday, when United welcome top-flight side Fulham to Bramall Lane in the fourth round of the FA Cup.
