Per Ryan Divish of the Seattle Times, Werth agreed to a minor-league deal with the Mariners on Tuesday. He is going to spend time in extended spring training "to get in shape" before reporting to Triple-A Tacoma.
The veteran outfielder, who turns 39 in May, spent the last seven seasons with the Washington Nationals. He hit .226/.322/.393 with 10 home runs and 29 runs batted in last season.
A combination of injuries and a platoon in the Nationals outfield limited him to 70 games in 2017. After hitting .282 in April, he did not hit better than .250 in any remaining month of the season. He finished 3-for-18 with one run during the Nationals' postseason run.
"I brought him here to shape us as a championship-caliber franchise," Nationals general manager Mike Rizzo said in October, per Mark Bowman of MLB.com. "Slowly, we've kind of morphed into a very professional organization. We have a protocol and a process. He has been an instrumental factor in getting us where we're at."
Werth's decision to extend his career is one predicated on his desire to win. His Nationals contract banked him $126 million; there's no financial incentive for him to return. But with a decade having passed since his lone World Series ring with the Philadelphia Phillies, he may be hoping to make a go at another title with the Mariners.
In January, Werth told Jerry Crasnick of ESPN he has "a few good years left" as an everyday player.
Mitch Haniger, Dee Gordon and Guillermo Heredia are penciled in as the Mariners' outfield starters while Ben Gamel recovers from an oblique injury. They also have Ichiro Suzuki and Kirk Nieuwenhuis able to come off the bench.