The 32-year-old has agreed a contract until the summer of 2016, ending his seven-year stint with Inter Milan.
Cesar, who has won 64 caps for his country, told the QPR official website: "I'm so happy and excited to be here.
"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."
Watch: Transfer Update: Thursday August 30th... More Videos
Cesar, who won the Champions League and five Serie A titles with Inter, was spotted on Tuesday at Loftus Road, where he was discussing terms ahead of Rangers' 3-0 Capital One Cup second-round victory over Walsall.
Manager Mark Hughes was thrilled to add a goalkeeper of 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 said on 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."
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 September 15.
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."
Cesar's arrival at QPR would more than likely mean Rob Green losing his number one spot after just three games following his summer arrival from West Ham.
Speaking after Tuesday night's cup game, Hughes said: "I haven't had a chance to really talk to him about it.
"Obviously, I was concerned with the preparation for the game, but certainly I will sit down with Rob and go through it.
"It is a challenge for everybody. What we're trying to do here is improve the quality of the group and that includes the goalkeeping position as well.
"I said I wanted to have two quality keepers this window if at all possible.
"Sometimes, when opportunities present themselves, you want to pursue them."