Diakite has agreed a four-year deal with the Loftus Road club having initially joined on loan last season.
The 23-year-old made nine appearances for Rangers last term, receiving a red card on debut against Fulham and scoring the winning goal against Arsenal.
QPR manager Mark Hughes told the club's official website: "Samba was a real find for us in January, so we're delighted he's joined us permanently.
"He's a very accomplished footballer and he's made to measure for the Premier League.
"He's got good pace, strength and ability on the ball and he's going to be a great asset for us going forward."
Diakite added: "It's a dream come true to sign this contract.
"I loved my time here on loan and I couldn't be happier to join QPR on a permanent deal.
"It's a fantastic club to play for and the owners have great plans for the future - I am just delighted to be part of it.
"This is a really exciting move for me."