The goalkeeper has penned a two-year deal at the Estadio da Luz and will hope to become their first choice between the posts following the summer departure of Jan Oblak.
The former Brazil international has signed a two-year deal at the Estadio da Luz and looks set to become the club's first-choice keeper following the departure of Jan Oblak to Atletico Madrid earlier this summer.
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Cesar joined QPR in the summer of 2012 but spent this past spring on loan at MLS club Toronto FC before playing for Brazil at the World Cup.
He was then deemed surplus to requirements at Loftus Road, where Robert Green is the current No. 1, with the club sanctioning his exit Tuesday.
Cesar made 27 starts in all competitions for The R's, with Benfica becoming the fourth permanent side he has played for following spells with Flamengo and Inter prior to his move to England.