Strasburg, 31, had four years and $100 million left on a deal he signed in 2016. He faced a Saturday deadline to opt out.
Strasburg enters free agency fresh off a regular season in which he posted a career-high 18 wins and led the National League with 209 innings pitched -- no small measure for a pitcher who has battled injury throughout his career.
He continued rolling in the playoffs. He became the first pitcher in major league history to win five games in a single postseason without a loss, his 47 strikeouts were tied for the second-most in a single postseason (Curt Schilling had 56 in 2001), and he posted a 1.98 ERA in six appearances overall.
The right-hander won two games in the World Series, including a pivotal Game 6 in Houston in which he became the first starter to go at least eight innings in a World Series since Matt Harvey did so in Game 5 in 2015. The following night, Washington won Game 7 and Strasburg became the first No. 1 overall draft pick to be named World Series MVP.
Opting out doesn't mean Strasburg won't return to Washington, which drafted him out of San Diego State in 2009. But he'll at least test the free-agent market, which will be highlighted pitching-wise by him and Gerrit Cole.
Strasburg, who is represented by Scott Boras, is a career 112-58 with a 3.17 ERA and 1,695 strikeouts over 1,428⅔ innings.
Just a few days after the Nats' World Series victory, the roster has already begun to change.
The club declined its $18 million option for next season on first baseman Ryan Zimmerman, 35, the only player who has been a member of every Nats team since the franchise moved from Montreal to Washington before the 2005 season. Zimmerman gets a $2 million buyout.
Washington also turned down a $9 million option on catcher Yan Gomes, who receives a $1 million buyout.
The team now must decide what to do with third baseman Anthony Rendon, who is a free agent after hitting 34 home runs, 44 doubles and a major league-best 126 RBIs this past season.
The Associated Press contributed to this report.