The 21-year-old has found first team opportunities hard to come by at Old Trafford this season, featuring only four times for David Moyes' side in all competitions.
"He has pace, power, technical ability and goes past people," Cardiff manager Ole Gunnar Solskjaer, himself a former United player, told reporters.
"He will give us that extra change of tempo in the final third, where we really need it. He will bring that little final bit to find pockets of space and produce a bit of magic."
Zaha becomes Solskjaer's sixth signing this month, joining Fabio, Kenwyne Jones, Magnus Wolff Eikrem, Mats Moller Daehli and Jo Inge Berget at the Cardiff City Stadium.
Cardiff, who are bottom of the Premier League with 18 points, travel to Norwich City on Saturday.
($1 = 0.6062 British pounds)
(Reporting By Michael Hann)
