Bolton manager Sam Allardyce believes the arrival of Cesar until the end of season will improve his defensive options.
The former Spain international, 29, is able to move outside the transfer window after having his contract at Levante cancelled.
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.
With Israel defender Tal Ben Haim out of contract in the summer and ready to move on, the Bolton boss has moved quickly to bring in cover.
Abdoulaye Faye and Abdoulaye Meite are the only other senior central defenders at the club.
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 said: 'I am aiming to be available in two weeks to help the team this season.'
Bolton are aiming to continue their push for a European place on Sunday when they face Tottenham at White Hart Lane.