The 36-year-old hit .205 in 54 games for the Orioles last season, and he joins fellow infielder Alex Cintron, first baseman Kevin Millar and left fielder Jay Payton as free agents in the first week of filing.
Castro, who also played seven games for the Reds in 2008, has played in the Major Leagues in every season since 1995. The veteran wrestled the starting shortstop job from a flotilla of rivals last year, providing steady defense for Baltimore down the stretch. Before Castro arrived, the Orioles cycled through Luis Hernandez, Brandon Fahey, Freddie Bynum and Cintron.
The Orioles are expected to find a new starting shortstop this offseason, either through free agency or trade. The position is regarded as a weakness within the organization, lending no immediate help in sight. Castro, a .228 career hitter with a .268 on-base percentage, will likely try to latch on somewhere with a Minor League contract and an invitation to Spring Training.