Diakite, 23, made nine appearances for QPR last season following his loan move from Ligue 1 Nancy in January.
"Samba was a real find for us in January, so we're delighted he's joined us permanently," QPR manager Mark Hughes said on the club website.
"He's got good pace, strength and ability on the ball and he's going to be a great asset for us going forward."
QPR finished 17th in the table last season, one point above the relegation zone.
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