General manger Mike Rizzo said Riggleman quit after the team denied his request for a new contract. Rizzo says Riggleman quit after he requested the team pick up the option on his contract for 2012 and the club declined.
Rizzo made the announcement Thursday afternoon following the Nat's victory. The Nats have won 11 of the last 12 games.
The team is expected to name an interim manager Friday before facing the White Sox. Rizzo says Riggleman told him he would not travel with the team to Chicago.
Riggleman is the second manager to resign this week. Florida Marlins manager Edwin Rodriguez quit on Sunday.