The 32-year-old has agreed a contract until the northern summer of 2016, ending his seven-year stint with Inter Milan.
"The ambition of this club was presented to me by the chairman and the coach and I couldn't have been more impressed.
"The desire to achieve things at this club is clear for everyone to see and I wanted to be part of that. I have come here to compete for the shirt and give my all."
Julio Cesar, who won the UEFA Champions League and five Serie A titles with Inter, was spotted on Tuesday at Loftus Road, where he was discussing terms ahead of QPR's 3-0 Capital One Cup second-round victory over Walsall.
Manager Mark Hughes was thrilled to add a goalkeeper of Julio Cesar's quality to his squad.
"It's fantastic that we've been able to attract a keeper of such outstanding ability to the club," the Welshman told www.qpr.co.uk
"When the opportunity to sign a player of his quality presents itself, you have to act quickly and we've managed to get the deal done.
"It was always my intention to bring two goalkeepers to the club this summer and now we've got fantastic competition in that department."
Hughes added: "He's an outstanding individual, as well as being an outstanding player.
"Not only will we benefit from his ability, but we'll also benefit from his experience and winning mentality."
Julio Cesar, who will wear the number 33 shirt, is likely to be in contention for the club's next home game against Chelsea - after the international break - on 15 September.
And the South American cannot wait to get started, adding: "The English Premier League is the best division in the world.
"Everyone wants to play here and I am no different. Now I just want to give my all to the club and show the fans what I am capable of.
"This is a really exciting move for me and I can't wait to get going."
Juventus has agreed to buy the remaining half of midfielder Kwadwo Asamoah's registration from Serie A rivals Udinese for $12.9 million.