"QPR are a club with huge ambitions to return to the Premier League and I wanted to be part of it," the player said.
"I had a really good feeling about QPR from the moment I arrived here. I’ve been welcomed by the club and I feel good here already.
"I’ve already integrated well into the group, so I’m looking forward to what will hopefully be a very successful and happy time here."
Former Milan centre-back Onyewu, 31, was available after leaving Sporting Lisbon in the summer.
The 6'5" stopper also spent time on loan at FC Twente and Malaga, where he made just two appearances last season.
Onyewu, who started his professional career in France with Metz and has also played for Standard Liege in Belgium, has 68 caps for the USA.
This is the second time Onyewu has played in England, having spent time on loan at Newcastle in 2007.
"This was an opportunity that was presented to me, that I was interested in, and I’m really pleased to be here,” he added.
"I want to help the club get back to the Premier League – that’s why I’ve come here.
"Harry (Redknapp) believes I can come in and help the team out and when you’ve got the backing of the coach and his staff, ultimately that tells me that I’ve made the right decision to come here and try this new challenge.
"England is a fantastic place to play and I can’t wait to get started here."