A person familiar with the deal said the left-handed Corbin has agreed to a six-year contract with the Nationals, pending a physical exam. Another person familiar with the terms said the contract is for $140 million.
The people confirmed the details of the agreement to The Associated Press on condition of anonymity Tuesday because nothing had been announced by the team.
Corbin's contract would include $10 million in deferred money, one of the people said.
Corbin, who has played his entire major league career for the Arizona Diamondbacks, joins a rotation in Washington that already includes three-time Cy Young Award winner Max Scherzer and Stephen Strasburg. Corbin was an NL All-Star selection in 2013 and this year; Scherzer has been picked for each of the past six Midsummer Classics; Strasburg has made it three times.
The 29-year-old Corbin is coming off his best season in the majors, going 11-7 with a 3.15 ERA while pitching 200 innings for the second time in his career.
He finished fifth in voting for NL Cy Young honors; Scherzer was second as he came up just shy of a third consecutive prize.
Corbin was considered the top starter available in an elite free-agent class and became the first player among them to sign a big money, multiyear deal this offseason.
Still on the market is right fielder Bryce Harper, the 2012 NL Rookie of the Year and 2015 NL MVP for the Nationals. Washington made a late-season offer to Harper in a bid to keep him, but that was not accepted.
Corbin recently made the rounds of free-agent suitors, including meeting with Nationals owner Mark Lerner and general manager Mike Rizzo.
The signing of Corbin - who had drawn interest from the New York Yankees and Philadelphia Phillies, among other clubs, and was expected to draw offers of at least $20 million a year - could be a signal that the Nationals are moving on from Harper.
After going 82-80 and failing to make the playoffs after two NL East titles in a row, Washington has been busy this offseason.
In addition to the agreement with Corbin, the Nationals already have added two catchers - Yan Gomes via trade, Kurt Suzuki in free agency - and two back-end-of-the-bullpen relievers - Kyle Barraclough via trade, Trevor Rosenthal in free agency.
Because Corbin declined a $17.9 million qualifying offer by Arizona and the Nationals will pay luxury tax this year, Washington would forfeit its second- and fifth-highest picks in next June's amateur draft and lose $1 million in international signing bonus allocation for 2019-20. Arizona would gain an extra pick after the first round as compensation.
AP Baseball Writer Ronald Blum contributed to this report.
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