The Mets, who had put him on waivers four days earlier, will get two players to be announced later from the Red Sox, Major League Baseball said on its Web site.
Wagner waived his no-trade clause in return for the Red Sox agreeing not to exercise his 2010 option while keeping the right to offer him salary arbitration.
That allows Wagner to become a free agent after the season and puts Boston in line to be compensated by any team that signs him.
The Red Sox reportedly are on the hook for the prorated portion of his $10.5 million 2009 salary for the rest of the season.
Wagner, 38, has 385 career saves, putting him within striking distance of John Franco, who holds the record for left-handers at 424.
He had reconstructive elbow surgery less than a year ago.
Copyright 2009 by United Press International.