The Championship club announced the deal for the former Italy Under-21 international on Tuesday.
QPR head coach Chris Ramsey said: “Gabriele’s a quality addition for us.
“We’ve got decent competition now in the back four and Gabriele’s experience, especially at a level where he was promoted with Watford last season, will be a big positive for us.
“I’m delighted we’ve been able to get this over the line before the deadline.”
Angella, who made 36 appearances in the Hornets’ promotion-winning season last term, added: “I am very happy about this big opportunity.
“I want to help the club this season and do my best here – this is a new page in the book of my career.
“I just want to play and improve and show the fans what I can do.”