Allardyce said: "With only three experienced centre-backs at my disposal, Cesar can make an impact in the last 11 games.
"I have been monitoring him for quite some time and he has some key attributes.
"He has the potential to strengthen our defensive unit once he reaches the necessary fitness."
Allardyce first tried to sign Cesar when the player was at Deportivo La Coruna but was unable to make much headway.
Allardyce said: "I am delighted to sign Cesar for the rest of this season.
"Once he became a free agent we moved quickly to secure his services."
Cesar has made more than 200 appearances in the Primera Liga, most of them with his first club Real Oviedo, and has been capped nine times, although not since 2004.