The Red Sox will also get Oakland's competitive balance draft pick while the Athletics received cash considerations in the trade, which bolsters Boston's historically weak outfield offense.
The deal, which took place hours before baseball's 4 p.m. ET non-waiver trade deadline Thursday, marks the end of Lester's storied stint with the Red Sox, who won two World Series championships with the left-hander in their rotation.
"My last conversation with him was in the clubhouse," Red Sox general manager Ben Cherington said. "He came in to clear some stuff out so I got a chance to go down and see him in person today. That was a thank you and an appreciation for everything he's done and a good luck.
"He's going to a new place and he's going to be pitching in big games down the stretch and he'll have an opportunity to win again. We'll be rooting for him."
The Red Sox made three other trades Thursday, including sending veteran starter John Lackey to the St. Louis Cardinals for outfielder Allen Craig and right-hander Joe Kelly, and a deal with the rival Yankees that sent shortstop Stephen Drew to New York in exchange for veteran infielder Kelly Johnson.
"We felt like what made the most sense for us was to try to focus on impact major league talent that is ready and we have a lot of good young players, we have strength in our farm system, so that is already a strength," Cherington said. "Although there were some prospect packages or prospects available to us that were very attractive, we wanted to add to the major league team and really give ourselves a head start on like I said building again and becoming as good as we can as quickly as possible.
"That guided us at least on the Lester and Lackey deals toward more proven major league players."
Lester, who is scheduled to become a free agent after this season, had publicly stated his desire to remain with the Red Sox. But contract negotiations stalled this past offseason, when Lester reportedly turned down Boston's four-year, $70 million offer.
Red Sox slugger David Ortiz, a teammate of Lester's since 2006, tweeted farewell to the ace pitcher on Thursday morning.
Wishing my man Jon Lester all the best, Red Sox Nation and the Large Father will miss you! pic.twitter.com/7uWpXw8XnH
- David Ortiz (@davidortiz) July 31, 2014Athletics general manager Billy Beane, who has been the featured subject of a best-selling book and hit movie but has never taken a team to the World Series, left little doubt he is all-in for October by making his second blockbuster trade for a starting pitcher in less than a month.
Beane surrendered his top two prospects to the Chicago Cubs on July 5 in a trade for All-Star Jeff Samardzija and veteran right-hander Jason Hammel. In Lester, Oakland is adding a pitcher with a glittering postseason resume, including a 0.43 career ERA in the World Series.
Beane followed up the Lester deal Thursday morning by trading left-hander Tommy Milone to the Minnesota Twins for outfielder Sam Fuld. Oakland has a 66-41 record, the best in the American League, but leads the second-place Los Angeles Angels by just 2 ½ games in the AL West.
"I believe that the deliberations leading to the two trades we did today were deeply analyzed, thoughtful and value-added," A's owner Lew Wolff said in an email. "The best trades are when all parties have potential benefits, and what Billy accomplished is an example of such a win-win. The calls I have received from other owners were very complimentary."
Lester will join a star-studded Athletics rotation that already includes Samardzija, All-Star Scott Kazmir, promising youngster Sonny Gray, Hammel and Jesse Chavez. Hammel, who has struggled badly since joining Oakland, could be in danger of losing his rotation spot with the arrival of Lester.
The Red Sox had been seeking what one industry source termed "a king's ransom" in prospects for Lester from other potential trading partners, which included the Pittsburgh Pirates, St. Louis Cardinals, Los Angeles Dodgers, Toronto Blue Jays and Baltimore Orioles.
But when those potential suitors were unable to satisfy their demands, the Red Sox changed course with Oakland and acquired a proven major leaguer in Cespedes, the two-time reigning Home Run Derby champion.
Cespedes addressed the trade on Twitter, thanking the A's and their fans:
Oakland, you gave me your heart and your love. Many fun, winning times in A's Nation. Thank you. New chapter for me. Remember #JustWinBaby
- Yoenis Cespedes (@ynscspds) August 1, 2014The A's gave me my first MLB home. Always great to me and my family. Grateful forever. Gracias #A's
- Yoenis Cespedes (@ynscspds) August 1, 2014The Red Sox had made finding a run-producing outfielder a priority; over the past month, they had scouted Matt Kemp of the Dodgers, as well. But Kemp was still owed $107 million over the next five years, and Dodgers GM Ned Colletti, in addition to giving Boston a firm no on his top prospects like outfielder Joc Pederson, made it clear he was not moving Kemp.
Cespedes has 17 home runs this season, three more than Boston's outfielders have combined to hit in 2014. He also possesses one of the game's most powerful throwing arms, which in combination with center fielder Jackie Bradley Jr. and right fielder Shane Victorino, should give the Red Sox a shutdown outfield defense.
Victorino, though, continues to have problems with his hamstring, and likely will return to the DL. Cherington said the Red Sox will "acclimate Cespedes to right field" with Victorino out.
"He's obviously a power bat who hits the ball out to all parts of the park and should benefit from Fenway," Cherington said. "He's going from a ballpark in Oakland that doesn't help right-handed power hitters quite as much as Fenway does so we're excited to have him. He's clearly someone we don't have. As we were going through the options, finding someone we didn't have was appealing to us."
At 28, the Cuban-born Cespedes is two years younger than Lester (Cespedes turns 29 on Oct. 18) and will remain under contractual control by the Red Sox for at least another year. Though he has less than three years of major league service time, Cespedes can become a free agent after the 2015 season through a clause in his contract that allows him to avoid salary arbitration and hit free agency.
"I think we just have to get to know him a little bit more, he needs to get to know us, but this is a guy in the prime of his career who is a dynamic player who does a lot of different things well," Cherington said. "Certainly a type of guy we could envision wanting to have here longer, but lets get him here and get to know each other first."
Cespedes has had a rough month of July -- with a slash line of .198/.221/.352/.573 -- and on-base percentages of .294 and .303 the past two seasons, which do not fit the profile of what the Red Sox usually are seeking. But he should combine with Ortiz and Mike Napoli to give the Red Sox a formidable middle-of-the-order presence -- and one who will regularly visit The Wall. Interestingly, 11 of Cespedes' 17 home runs have come in cavernous Oakland Coliseum.
But in dealing Lester, the Red Sox have paid a high price -- a homegrown staff ace who has proven durable and dependable and was enjoying arguably the best season of his career. He leaves Boston with a 110-63 record in parts of nine seasons with the Red Sox, and was 10-7 with a 2.52 ERA in 2014.
Scratched from a scheduled start Wednesday, Lester was 4-0 with a 1.07 ERA in his last eight starts with the Red Sox, with 54 strikeouts, nine walks and just one home run allowed in 58 2/3 innings.
But the Red Sox decided early on they would not pay the current market value for a premier starting pitcher. Their reported four-year offer in spring training was worth less than half the six-year, $144 million deal Max Scherzer, another impending free agent, turned down from Detroit this spring.
Lester vowed that he would postpone further negotiations and adhered to that pledge, even when Red Sox executives approached his agent, Seth Levinson, and asked to re-open talks.
Because Lester was traded in midseason, any team that signs him this coming offseason will not have to forfeit a draft pick as compensation, which should only increase the market for his services. Even this week, Lester said he would be open to returning to Boston, but there is little reason to believe the Red Sox would pay a premium over what they could have signed him for while he was still in Boston.
"He's an Oakland A and he's got a job to do for them," Cherington said. "When we get to the offseason we get to the offseason."
In losing Gomes, the Red Sox are giving up a valuable part-time player who came to symbolize the "Boston Strong" motif adopted by the team in the aftermath of last year's Boston Marathon bombings, posing with biceps flexed on an iconic Sports Illustrated cover last spring.
Gomes hit six pinch-hit home runs while a member of the Red Sox, one fewer than record-holder Ted Williams, and lived up to advanced billing as a platoon player who could hit lefties. He has a slash line of .302/.400/.431/.831 against lefties this season.
In Oakland, Gomes returns to a club where he helped lead a young team to a playoff spot in 2012, and is likely to keep an impressive streak of playing for postseason teams, having been on playoff teams with the Rays, Reds, Red Sox and the Athletics.
He leaves with a fond spot for Boston.
"I'm in a situation right now where I'm getting packaged up with the best pitcher in the game, heading over to the team with the best record in the game," Gomes said in a phone interview with Boston sports radio station WEEI. "I'm a little bit excited there, but at the same time, you talk about a soft spot in my heart, soft spot with some of the relationships I've made in Boston and this chapter, for the time being, has come to an end."
ESPN.com's Jerry Crasnick, ESPN Insider Jim Bowden, ESPNBoston.com correspondent Kyle Brasseur and The Associated Press contributed to this report.
Red Sox Deal Lester For Cespedes
Tim Kurkjian breaks down the trade that sends LHP Jon Lester to the Athletics for OF Yoenis Cespedes.
Lester Dealt To Athletics
ESPNBoston.com's Gordon Edes breaks down the deal that sends Yoenis Cespedes to the Red Sox for Jon Lester.