The centre-back, in the United States preliminary squad for the World Cup, has missed almost all Milan's season because of a serious knee injury but is now quite literally paying the club back for standing by him.
"Onyewu asked for, and received, a one-year contract extension from Milan, from June 30, 2012 to June 30, 2013," Milan, third in Serie A this term, said in a statement.
"During this extra year, Oguchi Onyewu, by his choice, much appreciated by the Rossoneri club, has asked not to receive any kind of salary. This is an exemplary gesture which deserves sincere congratulations."
The 28-year-old was in his first season with the Italian club when he tore a knee tendon in October playing for the U.S. in a World Cup qualifier.
He is in the preliminary squad for the June 11-July 11 tournament in South Africa despite only just recovering his fitness and not featuring for Milan in the Serie A run-in.
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