The deal includes an option for Deportivo to sign the 22-year-old permanently for 30 million euros at the end of the season.
Casado moves to the newly promoted La Liga side after finding his opportunities at Barcelona increasingly limited. He has made 75 senior appearances for the Catalan club but has fallen behind Rodri, Frenkie de Jong and Marc Bernal in the midfield pecking order.
The Spain international, who has two senior caps, can operate as a holding midfielder or right-back and brings considerable top-level experience to Antonio Hidalgo's squad.
His Barcelona record includes 49 La Liga appearances and 19 Champions League outings, while he has also collected two La Liga titles, one Copa del Rey and two Supercopa de Espana trophies.
Deportivo had been exploring a loan move for Casado before the transfer was completed, with the midfielder keen to remain in Spain and secure regular first-team football.
The Galicians have made an encouraging return to the top flight, drawing with Elche and Malaga before beating Valencia. They currently sit eighth with five points from their opening three matches.
Casado could make his Deportivo debut on Saturday when they travel to Villarreal for their fourth league fixture.
