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.
"[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."