Sources told MLB.com, ESPN and The Athletic on Monday that the veteran starting pitcher and the Nationals agreed to a one-year contract. The left-handed pitcher will fit into the starting rotation behind Max Scherzer, Stephen Strasburg and Patrick Corbin.
Lester's new pact is pending a physical. The deal features a mutual option for the 2022 season.
Lester, 37, has spent his last six seasons with the Chicago Cubs. He went 3-3 with a 5.16 ERA in 12 starts last season.
He allowed allowed a league-high 205 hits and had a 4.46 ERA in 31 starts in 2019. Lester posted a National League-best 18 wins in 2018 to earn his last All-Star nod.
He has a 3.60 ERA, 193 wins and 2,397 strikeouts for his career. The 15-year veteran has also suited up for the Boston Red Sox and Oakland Athletics.
Lester won the World Series with Boston in 2007 and 2013. He won another title in 2016 with the Cubs.