The Whites confirmed the deal via Twitter:
The 32-year-old had been with Real since 2015, having left the club as a youngster in 2007.
The Spaniard made just 43 senior appearances for Los Blancos and has not made any this season, having been back-up to both Thibaut Courtois and Keylor Navas at the Santiago Bernabeu.
While he has played relatively little, he has been present for a great deal of success during his time with Real:
He's now hoping to earn promotion with Leeds:
Spanish football expert Guillem Balague is an admirer of Casilla's:
Marcelo Bielsa's side are top of the Championship by four points, and only Middlesbrough have boasted a better defensive record this season in England's second tier.
Bailey Peacock-Farrell, 22, has kept 10 clean sheets in his 26 Championship appearances this season, but Casilla will give the Whites a more experienced alternative to call upon in the run-in.
It could take him some time to get up and running having not played since last season, but he could play a significant role in Leeds' promotion push.