Werth will join the organization in Peoria, Arizona, and will get in shape in extended spring training before joining Triple-A Tacoma, according to reports.
The signing is contingent on Werth's passing a physical.
In 2017, Werth hit .226 with 10 home runs and 29 RBIs in 70 games for the Washington Nationals. He missed almost three months with a broken left metatarsal after fouling a ball off his foot in early June.
During his seven years in Washington, Werth battled through injuries and averaged 115 games per season. A career .267 hitter, the 38-year-old outfielder ranks fourth in Nationals history in home runs (109) and RBIs (393). Known for his ability to extend at-bats, Werth averages 4.40 pitches per plate appearance for his career.
A 15-year veteran, Werth spent time with the Toronto Blue Jays and Los Angeles Dodgers before landing in Philadelphia, where he won a World Series with the Phillies in 2008, then made his lone All-Star appearance the following year. Before the 2011 season, he signed a seven-year, $126 million contract with the Nationals.
In 2012, when the Nats reached the playoffs for the first time in eight seasons since moving from Montreal to Washington, Werth hit a walk-off home run against Cardinals pitcher Lance Lynn in Game 4 of the National League Division Series. The dramatic homer came on the 13th pitch of the at-bat and tied the series. The Nationals eventually lost the NLDS to St. Louis, the first of four consecutive division-series defeats during Werth's tenure in Washington.
ESPN's Eddie Matz contributed to this report.