The former Blackburn centre-back was one of the west Londoners' key targets for transfer deadline day as Harry Redknapp looks to shore up his defence.
That deal is now complete, with Samba joining for an undisclosed club record fee, subject to the player receiving international clearance.
The Congolese defender, who was with Blackburn until last February, will wear the number five shirt at Loftus Road and fills the void left by Ryan Nelsen after his decision to become Toronto FC head coach.
"This is an unbelievable signing," Redknapp told the club's official website, www.qpr.co.uk.
"[Chairman] Tony Fernandes deserves a lot of credit for this one - he has worked so hard on bringing him in.
"I was speaking with him a month ago and he asked me who I would want to sign if Ryan Nelsen left, he asked me to give him a couple of names.
"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 joins a side bottom of the Premier League and four points adrift of safety - but believes QPR can get out of their current predicament.
"I'm delighted to be back in the Premier League," he said. "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.
"I know what is going on with the project here. There is a very important fan base, I've played at Loftus Road before and it's fantastic.
"The club is ambitious and I'm sure it's a good challenge for me. If I can be a part of it I will be very proud, it will be a great achievement for me.
"I wouldn't be here if I didn't believe we could get out of trouble."
Now Samba has completed his move to west London, Redknapp can focus on bringing in additional new faces on deadline day.
The QPR boss is targeting another striker, with Stoke's Peter Crouch and West Brom's Peter Odemwingie amongst those targeted.
Redknapp is also looking to former club Tottenham for additions, with interest in Jermaine Jenas and David Bentley.