The 20-year-old, who moved to Wearside from in Ipswich in 2011 for £8m has represented England at all levels up to Under-21.
The youngster has had two loan spells at Sheffield Wednesday since his move to the Premier League, including one this season. Wickham scored eight goals in 11 games for the Owls, including two against Leeds in their 6-0 defeat at Hillsborough.
He becomes the second loan signing in a week, after Jack Butland joined last week on loan from Stoke. Wickham promises to give Leeds another option up top and brings some real quality to Elland Road.
Striker Ross McCormack needs help up top and Wickham could be the man to link up with him and fire Leeds up the table.
Leeds' play-off hopes hang in the balance, but Wickham insists he is eager to help Leeds finish inside the top six.
He told the club's official website:
"I need to be playing football. I'd been playing at Wednesday and I knew there was some interest in me, but I just needed to focus on playing football.
"Once I heard Leeds were interested in signing me there were no other choices and no doubt I wanted to come. I played against Leeds earlier this season with Sheffield Wednesday. 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.
"I just want to play football and do the best I can, and hopefully push the team into the promotion picture."