Wickham, an England under-21 international, came through the ranks at Ipswich Town before moving to Sunderland in 2011.
However, having failed to secure a regular first-team place at the Stadium of Light, making just one start for the club this season, he has spent much of the current campaign on loan at Leeds' Championship rivals Sheffield Wednesday.
Wickham, who was also loaned to Wednesday for a spell last season, scored eight goals in 11 appearances for Stuart Gray's side in his most recent stint with the club.
Leeds manager Brian McDermott will hope the 20-year-old can provide extra firepower and fire his side, currently in 11th, to the play-offs.
"We are delighted to have signed Connor until the end of the season," McDermott told Leeds' official website.
"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 is the second loan signing made by McDermott in recent weeks, with goalkeeper Jack Butland arriving from Stoke City last Thursday.
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