Jon Heyman of the MLB Network reported Friday it's a one-year, $4.25 million deal.
This will be the third team for Pillar since the start of last season. The 31-year-old began 2019 with the Toronto Blue Jays, but the emergence of Teoscar Hernandez as an everyday player made him expendable for the rebuilding organization.
The San Francisco Giants acquired Pillar on April 2, and he became a staple in their outfield. His offensive production lagged because of a .287 on-base percentage between the two clubs, but he added surprising power to the lineup with a career-high 21 homers.
Even though Pillar is a limited offensive player, he led the Giants or tied for the lead in homers, hits (157), doubles (37) and stolen bases (14) in 2019. Pillar also drove in a career-high 88 runs, and his .259 batting average was his best since 2016.
He played 222 innings in right field for the Giants, which allowed him to have a solid rebound year on defense. He had two defensive runs saved and a 5.4 ultimate zone rating per 150 innings at the position, per FanGraphs.
The Giants decided not to tender Pillar a contract for next season by the Dec. 2 deadline, making him a free agent for the first time.
"It's a tough decision because he had a nice year for us and clearly connected with the fans with his production and his style of play," Giants president of baseball operations Farhan Zaidi told reporters in December. "We certainly wish him the best, but with where we are as an organization, we've got to be in a little bit of a development mode."
This free-agent class wasn't loaded with outfielders, especially ones like Pillar with a strong track record on defense and speed to create problems on the bases. The power he displayed last season also boosts his value as he enters the next phase of his career.
The Red Sox will be able to use Pillar in center field or right field. He could also be an option off the bench in the later innings because of his ability to steal bases.
After dealing outfielder Mookie Betts to the Los Angeles Dodgers in a trade that brought back promising outfielder Alex Verdugo, the Red Sox needed outfield balance.
Projected starters Verdugo, Andrew Benintendi and Jackie Bradley Jr. are all left-handed hitters, whereas Pillar is a righty. He is the perfect candidate to serve as the fourth outfielder or even a part-time starter, depending on matchups, and his presence will allow J.D. Martinez to continue to focus primarily on being the designated hitter.