Castro has signed with the yet-to-be-named Atlantic League based in Frederick, Maryland.
Castro was released by the Nationals on Thursday after his domestic violence suspension was completed, Britt Ghiroli of The Athletic reports.
Castro's administrative leave was extended by seven days Friday, Jessica Camerato of MLB.com reports.
General manager Mike Rizzo said Tuesday that he does not expect Castro to return to the Nationals this season after allegations of domestic violence placed the infielder on administrative leave last week, Jesse Dougherty of The Washington Post reports.
Castro was placed on administrative leave Friday while the leave investigates an alleged domestic violence incident, Britt Ghiroli of The Athletic reports.
Castro went 2-for-3 with a walk and a run scored in Tuesday's 7-4 loss to the Padres.
Castro went 2-for-4 with a double and one RBI in Sunday's 5-1 loss to the Dodgers.
Castro went 1-for-4 with a double Tuesday in the Nationals' 3-2 win over the Phillies.
Castro (personal) was reinstated from the restricted list Friday, Matt Weyrich of NBC Sports Washington reports.
Castro was placed on the restricted list for an undisclosed reason Wednesday, Maria Torres of The Athletic reports.