New York gets a player to be named and will pay the Brewers cash to cover a portion of the $4,041,530 remaining in Walker's $17.2 million salary. He can become a free agent after the World Series.
The 31-year-old is hitting .264 with 10 homers and 36 RBIs in 73 games this season after batting .282 with 23 homers and 55 RBIs in 2016, when he had season-ending back surgery on Sept. 8.
Milwaukee is third in the NL Central, two games behind the first-place Chicago Cubs.
"I'm excited to be back in a playoff race," Walker said.
New York had expected to contend but is 52-62. The Mets previously traded outfielder Jay Bruce, first baseman Lucas Duda and relief pitcher Addison Reed.
"This series of transactions opens up the opportunity for us to observe some new players," Mets general manager Sandy Alderson said. "When things don't go the way they were expected to go, then you have to shift direction and do the best you can to change direction."
The Mets acquired Walker from the Pittsburgh for pitcher Jon Niese on Dec. 9, 2015, after allowing Daniel Murphy to become a free agent. Murphy has gone on to have two All-Star seasons with Washington.
Alderson praised Walker's performance in New York.
"He was an excellent player," Alderson said. "A first-rate guy. Very important in the clubhouse. I'm very glad we had him for the time we did and I think he distinguished himself as a Met."
Walker said he is looking forward to playing in Milwaukee, where he has batted .286 with 11 homers and 31 RBIs in 51 games at Miller Park. He expects to arrive in time for the Brewers' game against Cincinnati on Sunday.
"I've always felt pretty good there," he said.
Curtis Granderson, like Walker, Bruce, Duda and Reed, is eligible for free agency. Alderson hinted at the possibility of more deals, saying: "We still have inventory."
AP freelance writer Andrew Grunman in Milwaukee contributed to this report.
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