"The ability to get a Chris Sale doesn't come along very often,'' Boston president of baseball operations Dave Dombrowski said.
"When you trade a pitcher of Chris Sale's ability, it can only be because we were motivated by an impactful return of young talent, and we have more than accomplished that," White Sox general manager Rick Hahn said in a statement. "We believe each of those players can be part of a quality core of future championship caliber White Sox teams."
Dombrowski said that the trade talks between the two sides began in earnest last Friday after baseball's new labor deal was reached, and they heated up from there.
"I think it's a situation where if you have a chance to win you go for every opportunity you can." Dombrowski said in a news conference. "Any time you can make the playoffs, you go for it."
Sale, 27, is coming off his fifth straight All-Star season in 2016, when he finished 17-10 with 233 strikeouts in 226⅔ innings pitched.
"I really appreciate the White Sox fans' support through the years and can't wait to come back to Chicago and see everyone," Sale wrote in a text to MLB.com. "I have a lot of great memories in Chicago, and a piece of me will always be in that city."
"The best thing is that we don't have to face him anymore," Red Sox manager John Farrell said. "This is a reminder of how aggressive our ownership and Dave is going to be to try to give us a chance to win every time we take the field."
Boston inherits a very team-friendly deal that will pay Sale $12 million in 2017, with club options of $12.5 million for 2018 and $13.5 million for 2019.
Sale is 74-50 with a 3.00 ERA and 1,244 strikeouts in 1,110 innings pitched over his seven seasons with the White Sox.
"He pitches with an edge," Farrell said. "I think there's a persona he projects when he's on the mound. That edge and competitive nature he has makes him a premium-type player."
Newly retired slugger David Ortiz welcomed Sale on Instagram, making a joking reference to returning for Boston next season that read, "My god?????? my boy sale to Btown? You guys got me thinking ???"
This year, the relationship between Sale and the White Sox became extremely strained.
Sale was suspended for five days without pay for destroying collared 1976-style uniforms the team was scheduled to wear July 23, saying they were uncomfortable. He lost $250,000 of his $9.15 million salary and also was fined about $12,700 -- the cost of the tattered jerseys. He blamed manager Robin Ventura for not defending his players.
During spring training, Sale was quite vocal about the decision to limit the time teammate Adam LaRoche's son was allowed in the clubhouse. That flap led to hard feelings all around, along with LaRoche's retirement.
The White Sox went 78-84, and haven't made the playoffs since 2008.
Boston went 93-69, then got swept by Cleveland in the AL Division Series and finish out the career of Ortiz.
The Red Sox invested heavily in Moncada two years ago, signing him out of Cuba for a record $31.5 million. They moved him to third base this season, and he made his major league debut late in the season. Although he revealed himself to be not yet ready for the big leagues -- he went 4-for-19 with 12 strikeouts -- the Red Sox still believe he has the ability to become an All-Star.
The 20-year-old Kopech, meanwhile, wowed scouts in the Arizona Fall League with a fastball that registers in the triple digits. He posted a 2.25 ERA in 11 starts for Class A Salem.
Basabe, also 20, hit .264 with 53 RBIs in Class A. Diaz, a 22-year-old righty, went 2-5 with a 3.88 ERA in relief in Class A.
ESPN Staff Writer Scott Lauber and The Associated Press contributed to this report.