Oakland will need to make a corresponding move to free up a roster spot for Riley.
Riley, 28, is a former fourth-round pick of the Redskins back in 2010. He was in the final year of his three-year, $13 million contract and set to make a $4 million base salary in 2016 when the Redskins cut him loose in the final week of the season.
Riley was still working his way back from a foot injury he sustained back in December of last year when he was released.
In 2015, Riley appeared in nine games and recorded 44 tackles, two interceptions and three pass defenses for the Redskins.