The 32-year-old brings a wealth of experience to the Championship club, having previously spent time on the books at AC Milan, Sporting Lisbon, Twente and Malaga.
He enjoyed arguably the most successful spell of his time in Europe with Club Brugge, playing 139 games from 2004 to 2009 for the Belgian giants – a league familiar to current Addicks boss Bob Peeters, who played and managed there for much of his career.
Onyewu will also be familiar to fans of Newcastle United and Queens Park Rangers, having enjoyed short spells with both Premier League sides.
The American played 11 games for the Magpies during a loan spell from Liege in 2007, while he was also on the books at QPR in the 2013-14 season, though he ultimately failed to feature for Rangers during his time at the club.
Onyewu was most recently with Sheffield Wednesday, playing 18 games for the Owls last term as part of a six-month stay with the club.
He will now look to impress Peeters enough to earn a deal until the end of the season, with Charlton chasing promotion back to the Premier League this season.
Onyewu is most famous for playing an entire season with AC Milan, without getting paid, after picking up an injury that ruled him out for an entire campaign.
He also clashed with then-Rossoneri teammate Zlatan Ibrahimovic during training, before the pair made up.