The centre-half, who left Blackburn for Russian side Anzhi Makhachkala a year ago, joins Rangers on a four-and-a-half-year contract.
Samba is a ready-made replacement for his former Ewood Park team-mate Ryan Nelsen, who has left for the MLS, and Harry Redknapp gave chairman Tony Fernandes credit for pulling off an "unbelievable" deal.
"Tony has worked so hard on bringing him in," Redknapp said. "I was speaking with him a month ago and he asked me who I would want to sign if Ryan Nelsen left.
"I said to him, 'Well, these aren't possible to get but if you're asking me who I'd have, I'll give you a couple of players.' One of them was Chris Samba.
"The next thing I know he's telling me that he's working on bringing him in! It's amazing.
"Chris is just what we need. He's a monster. Great in the air, quick, a leader, strong, fantastic in both boxes, hard as nails. He's a proper centre-half."
Samba said: "I'm delighted to be back in the Premier League. It's the best league in the world to play football.
"I've been linked with the club for a long time. With Harry here though, he is really passionate and he wants to do something with the club.
"Unfortunately things haven't gone the way the club would have wanted so far this season but there's still a lot of time and I'm sure we can get out of it."
Fernandes has now broken QPR's transfer record twice since the start of the month having paid £8m for Loic Remy earlier in the window.