The 28-year-old will play for free during the year that have been added to his deal.
Onyewu has been struggling with a knee injury this season.
A club statement said: “Onyewu, who has been out for virtually the whole season due to injury, but who at this time is getting ready to face the World Cup adventure in South Africa with his national team, has become a star today with a significant gesture from a personal and professional standpoint.
“Onyewu has requested and obtained a contract extension from June 30, 2012 until June 30, 2013.
“With respect to that year, as was his wish, which has been appreciated by the club, Onyewu has asked not to receive any type of wages.”
