The Andalusian club made the versatile American defender its third signing of transfer deadline day, having wrapped up deals for Roque Santa Cruz and Javier Saviola already.
The U.S. international can play in several positions but thrives in a central defensive role and will be added to the ranks already at coach Manuel Pellegrini's disposal.
The capture of Onyewu rounds off a busy day at La Rosaleda, as South American strikers Javier Saviola and Roque Santa Cruz were brought in on deadline day to help fill the void left by Jose Rondon's departure.
Onyewu holds a Belgian passport from his time at Standard Liege, and thus does not infringe upon Malaga's non-EU allowance.
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