The 27-year-old has moved across London for the rest of the season and is in line to play against Burnley in the Premier League.
"We're really pleased to bring Mauro in. He's a good footballer, someone with great ability who can play as a number 10," QPR manager Harry Redknapp said in a club statement.
"I first saw him when he was on loan at Birmingham quite a few years back and I actually tried to take him to Portsmouth back then. He's a more than capable player, he's already shown that, and hopefully he can come in here and do well for us."
Zarate has struggled to nail down a place in West Ham's first team, scoring twice in eight appearances since arriving from Argentine club Velez Sarsfield.
"I need to play and Queens Park Rangers have given me this chance to show what I can do," he said.
"I am fit and I hope I can be involved on Saturday."
QPR are 16th in the Premier League, two points ahead of second-bottom Burnley.
(Reporting by Martyn Herman, editing by Ed Osmond)
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