Pillar will provide the Red Sox with much-needed depth in the outfield after the loss of Mookie Betts via trade to the Los Angeles Dodgers and the uncertain status of Alex Verdugo, whom Boston acquired in the deal.
Verdugo may miss the start of the regular season because of a back injury that affected him toward the end of the 2019 campaign, according to the Boston Globe. He didn't play after Aug. 4 due to the back injury.
Pillar started last season with the Blue Jays before he was traded to the San Francisco Giants as part of a four-player swap in April. In the 2019 campaign, the 31-year-old outfielder posted career highs in multiple offensive categories, including home runs (21), RBIs (88), runs scored (83), triples (three) and slugging percentage (.432).
Piller, who earned $5.8 million last season, wasn't tendered a contract by the Giants after the season. He would have become eligible for arbitration and likely could have commanded about $10 million.
The Blue Jays selected Pillar in the 32nd round of the 2011 MLB Draft. In 851 career regular-season games, he has a .261 batting average with 76 home runs and 318 RBIs.