Bench coach McLaren will step in as interim manager for the Nats' road trip that starts Friday against the Chicago White Sox, but he's not expected to fill that role for the remainder of the season.
McLaren previously served as the manager of Seattle for parts of the 2007 and 2008 seasons. When he was fired there, he was replaced by Riggleman.
Nats general manager Mike Rizzo says Riggleman quit the Nats after he requested that the team pick up the option on his contract for 2012 and the club refused.
Riggleman has been seeking more job security for some time. He had a $600,000 contract this year and the Nats held a team option for 2012 -- also at $600,000.
Rizzo says the Nationals were stunned by the move.
The news follows the team's sweep of its series against the Seattle Mariners. They've won 11 of their last 12 games.
The first pitch of the White Sox is scheduled for 8:10 p.m.