The 31-year-old centre-half, who has won 68 caps for Jurgen Klinsmann's side, was a free agent after leaving Queen's Park Rangers at the end of his short-term contract there.
Onyewu joins the Owls until the end of the current campaign, but will not be available for Tuesday's FA Cup replay with Macclesfield.
He also counts Standard Liege, AC Milan, Newcastle and Malaga among his former clubs.
His arrival caps what has been a superb day for the South Yorkshire outfit after they battered Yorkshire rivals Leeds 6-0 in Saturday's lunchtime kick-off.
- Sports & Recreation
- Jurgen Klinsmann