The England U21 international, who returned to Wearside only recently after a spell at Sheffield Wednesday, heads back to the Sky Bet Championship for the rest of the season.
Wickham scored eight goals in 11 games for Wednesday and was used twice as a substitute by Sunderland earlier this month.
But Gus Poyet has sanctioned the switch to Elland Road, where Wickham will join up with fellow loanee and U21 colleague Jack Butland.
Leeds manager Brian McDermott told his club's official website: "We are delighted to have signed Connor until the end of the season.
"There was a lot of competition for his signature, but he was very keen to come and play at Leeds United.
"He is another young English player who has everything in front of him."
Wickham said: "I need to be playing football. Once I heard Leeds were interested in signing me there were no other choices and no doubt I wanted to come.
"The potential here is big. There are some players here that I know already, and the team is looking to push towards the play-offs. Hopefully I can help with that."
Managing director David Haigh said: "We're delighted to bring another quality player into the club: Jack showed why we were so keen to sign him with a terrific performance at Middlesbrough on Saturday, and Connor is another young player with a similar pedigree.
"Brian has made no secret of the fact that he has been looking to strengthen, and we're delighted we've been able to support him with two key acquisitions to the squad."