Castro, a four-time All-Star selection, recorded a .270 batting average with 22 home runs and 86 RBIs last season for the Marlins. After the season, Miami declined his $16 million option and instead paid a $1 million buyout on Nov. 1.
The 10-year veteran made 42 starts at third base with the Marlins in 2019, the first time he played the position. According to MLB Media, the Nationals are expected to utilize the versatile infielder at his typical spot at second base.
Castro played the first six seasons of his MLB career with the Chicago Cubs before spending two seasons each with the New York Yankees and Marlins. He has a .280 career average with 133 homers, 636 RBIs and 89 stolen bases.