He will receive $154 million during the deal, multiple media outlets, including ESPN, reported. Gonzalez receives a $6 million signing bonus, $21 million a year from 2012 to 2016 and
$21.5 million a year in 2017 and 2018, sources with knowledge of the deal told ESPN The Magazine's Buster Olney. Sources also said the deal has a partial no-trade clause.
Gonzalez said signing a long-term deal lets him focus on baseball: "Let's forget about everything else, just focus on winning and being part of the team."
Entering Friday's games, the Red Sox were 2-9, the worst record in baseball.
"We are disappointed. Never want to start this way," Gonzalez said. "We have faith in ourselves. We're going to turn this thing around."
The Red Sox acquired Gonzalez from the San Diego Padres in the offseason. The parameters of the deal were discussed at the time, but by waiting until after the season started, the Red Sox saved luxury-tax money.
A three-time All-Star, Gonzalez is in his eighth season. He has a career average of .282 with 169 home runs.
The contract is the second-largest in team history and the largest agreed upon by the team's current ownership. The $160 million contract signed by former Red Sox outfielder Manny Ramirez was reached while the team was owned by the Jean Yawkey Trust.
The Red Sox have made two major commitments this year -- to Gonzalez and Carl Crawford (seven years, $142 million). Red Sox general manager Theo Epstein said it does not signal a fundamental shift in how the club conducts its business.
"The reason you have discipline, the reason you have payroll flexibility, the reason you develop young players who are on affordable deals or on team-friendly long-term deals, the reason you do all those things is so when the right player comes along at the right time, you can make a commitment," he said.
"In our minds, Adrian was the right player at the right time."
Epstein says these deals are the exception, not the norm.
"We went seven years, eight years without doing a nine-figure contract," he said. "Not that we didn't try at times [Mark Teixeira, for example] but we always have our limits and we're always willing to walk away. Hopefully, that discipline puts us in a position to bet on the right player or right players."
Epstein expressed confidence that Gonzalez was worth the risk.
"If you're going to make this kind of commitment, I think you have to be very comfortable with not only the player but also the person," Epstein said. "If you're going to bet on one player, we're very comfortable betting on Adrian Gonzalez."
Despite the size of the contract, "that's a pretty low risk," manager Terry Francona said. "He gets it. He's a solid, solid teammate."
Epstein praised Gonzalez for how well he's fit in with his new team and the leadership he's displayed during the poor start.
"Adrian has really impressed everybody by being so engaged in every aspect of the game. He's not one of these great players who just shows up to hit four times a day," Epstein said. "He's been really active on defense, active in the clubhouse, involved in everything that's going on."
Gonzalez also has stayed optimistic as the team that was picked by many to win the World Series matched its record for its worst start in the first 11 games.
"I'm a very positive person," Gonzalez said. "I just keep telling everybody, 'Have fun, play your game and things will turn around.' ... One thing that we'll always have is that talent-wise we're going to be better than the other team, so just focus on that."
Gonzalez could have played out this season and become a free agent, but wanted the certainty of knowing where he would be.
"I'm a person that really likes to just focus on winning," he said. "When the focus isn't on the team, from my personal standpoint, that's not where I want to be. I want to be where 'let's forget about everything else and let's just focus on winning and being part of the team.'"
Gonzalez underwent offseason shoulder surgery and didn't start swinging a bat until several weeks into spring training.
"The last piece of the puzzle was Adrian's return to health," Epstein said. "He promised he'd be ready by Opening Day. He absolutely was ready by Opening Day and it made all the sense in the world to move forward" with the contract.
"I aged 100 years," said John Boggs, Gonzalez's agent, "but Adrian is fine with it. Usually the peripheral people are the ones, I guess, [who] carry a lot of the stress."
Senior writer Buster Olney covers MLB for ESPN The Magazine. Information from ESPNBoston.com's Gordon Edes and The Associated Press was used in this report.