The 6-foot-10, 245-pound Oberto averaged 3.6 points and 3.9 rebounds in four seasons with San Antonio. The Spurs traded Oberto to Detroit this summer as part of a five-player deal; the Pistons then waived Oberto.
Oberto helped the Spurs win the 2007 NBA title and Argentina win a gold medal at the 2004 Olympics.
"Fabricio is a proven winner with extensive playoff experience," Wizards president Ernie Grunfeld said. "He is a skilled big man and a rugged veteran."