The 26-year-old Chilean has received a work permit/governing body endorsement from the Football Association and is currently back in Italy to obtain his working visa.
Rangers boss Harry Redknapp told www.qpr.co.uk: "He's a top player and we're delighted to have him here.
"He showed his quality in the World Cup for Chile and it's a great loan signing for us.
"He's a versatile player, he's full of energy. I'm confident he'll be a really big asset for us this season"
Isla, who has made 51 appearances for his country, added: "I am very happy to be here - this is the start of an exciting new adventure for me.
"Everybody knows that the Premier League is the greatest league in the world and the QPR proposal is very interesting and one I am very excited about.
"I hope this coming season is going to be great for both the club and myself. I will give 100 per cent in every game to help QPR in the Premier League."
Earlier on Tuesday, the 47-cap Chile defender completed his medical at Loftus Road and tweeted news of his move - before later removing the post.
Redknapp's fourth summer signing follows just a day after QPR paid a fee believed to reach £6million to prise Jordon Mutch away from Cardiff.
Mauricio Isla will join QPR on loan deal from Juventus
QPR seem to have learned from their mistakes last time and are targeting quality over quantity. Isla is an excellent player, and something of a coup for the Hoops: he joins hugely the experienced Ferdinand and exciting young English talents like Mutch and Caulker.
Isla gives them much needed steel in midfield, and should form a good partnership with Joey Barton. A top striker and QPR could be the finished article.
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