Jon Morosi of MLB Network reported the contract is for five years and worth $110 million. Morosi noted the contract is "front-loaded prior to opt-out."
The New York Post's Joel Sherman supplied the likely financial consequences for Boston:
Martinez, 30, is coming off a career-best campaign that he split with the Detroit Tigers and Arizona Diamondbacks. He hit .303/.376/.690 with 45 home runs and 104 runs batted in despite missing 43 games. The struggling Tigers traded him midseason to Arizona, which earned a wild-card berth thanks in large part to Martinez's starmaking run.
Diamondbacks manager Torey Lovullo was blown away by Martinez's approach to the game.
"The thing that people don't see is what he does behind the scenes," Lovullo told reporters. "There's notes in a notebook. There's video and studying. There's tendencies and habits. There's constant practice and perfection of the swing. And it translates."
Martinez said in September that he would "love" to be a Diamondback over the long term, but his main focus was being on a contending team.
"I think one of the most important things in my free agency is I want to be on a team that's relevant—a team that's good, a team that's in it," Martinez told Nick Piecoro of AZCentral. "I don't want to be on a team just because [it offers the most money]. I want to be on a team that's got a chance. That's what's fun. That's what you play the game for."
FanGraphs ranks Martinez as the 13th-best outfielder in baseball since 2014. He's tied for third in wRC+ with Bryce Harper during that time.
Boston will be getting a player who is wildly productive when healthy but has had trouble staying on the field. He's missed at least 30 games in all but one of his six MLB seasons.
The Red Sox are hoping Martinez can stay healthy enough to improve their weak middle of the order. Boston finished with the fourth-worst home run total in baseball last season and saw its bats go silent during an ALDS loss to the Houston Astros.
Boston was briefly in on the Giancarlo Stanton sweepstakes and made adding a power bat a priority after the reigning NL MVP was traded to the rival Yankees.