The 25-year-old has joined the West London side after weeks of negotiations for an undisclosed fee.
"This was the right time for me to join a club like QPR," Onuoha told 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."
QPR boss Mark Hughes, who worked with the defender during his time at Manchester City, has welcomed the club's new addition.
"Nedum is a player I know very well," said Hughes. "He did very well for me at Manchester City and played a number of games, probably more than under any other manager.
"He is very versatile. He can play centre back or right back, and has good pace. He is a very good addition to the squad."
"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."
Onuoha could be in contention to make his QPR debut in the side's FA Cup fourth-round tie against Chelsea on Saturday.