After an unsuccessful spell at QPR, Onyewu will look to boost his slim World Cup chances at Sheffield Wednesday.
The 31-year-old joined QPR in October on a short-term deal, but failed to appear in a single match for Harry Redknapp's side.
The U.S. international has found playing time hard to come by in recent seasons, as he made just nine total appearances while on loan with La Liga outfit Malaga in the 2012-13 campaign.
Wednesday currently sits in 19th place of the English League Championship, four points above the relegation zone. Onyewu would be the fourth American to play for the Owls, after John Harkes, Frank Simek and Stuart Holden.
Onyewu has 68 caps for the U.S. national team, including two in 2013, most recently in a 4-1 Gold Cup win over Cuba in July.