The 25-year-old had been little more than a fringe player at City and now links up with former manager Mark Hughes at Loftus Road.
Onuoha becomes the recently-appointed QPR manager's second signing following the recent addition of AC Milan's Taye Taiwo and moves for an undisclosed fee, which Press Association Sport understands is in the region of £2.5million.
Hughes told the club's official website: "Nedum is a player I know very well.
"He did very well for me at Manchester City and played a number of games, probably more than under any other manager."
"This was the right time for me to join a club like QPR," Onuoha added on the club's official website.
"The manager obviously played a massive factor in my decision to come here. I've worked with him before and he always wants players around him who are open and honest.
"He wouldn't have come here if he didn't believe that there was a squad of players here that can achieve the goals he has.
"The ambition of the owners was really appealing. This is my club now and I'll give everything to help us succeed."
Onuoha could make his QPR debut in Saturday's FA Cup fourth-round tie against west London rivals Chelsea, although Hughes has previously indicated he is willing to give the versatile defender time to settle in due to his recent lack of first-team football.
Hughes added: "Nedum is very versatile. He can play centre-back or right-back, and has good pace. He is a very good addition to the squad."
Hughes added: "It's important that we are able to attract good players here.
"Nedum had other options but his experience with myself and the other guys here was a big factor in why he has come to us. I am pleased about that."