Zarate's move has been in the pipeline since the back end of last month and he will now join up with Harry Redknapp's men until the end of the current campaign.
The Argentine has struggled to establish himself since arriving at Upton Park from Velez Sarsfield during the close-season, but Redknapp is confident Zarate will strengthen his squad.
"We're really pleased to bring Mauro in," he said. "He's a good footballer, someone with great ability who can play as a number 10.
"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 is glad of the chance to play regular football, having made just five Premier League starts for West Ham.
"I am happy," he said. "I need this, I need to play and Queens Park Rangers have given me this chance to show what I can do.
"The Premier League is the best in the world at this moment. Does my style suit the Premier League? Maybe. We will see.
"When I met the manager he told me he wanted to sign me for Portsmouth before, but in that moment I went to Lazio. But now I am here and will play for him.
"I will try to do different things with the ball, some skills to make the difference for the team, for me and for the other players. I am fit and I hope I can be involved on Saturday."
QPR visit fellow relegation rivals Burnley in the Premier League this weekend.
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