ESPN's Jeff Passan first reported the trade Saturday evening on Twitter, and the club officially announced the trade several hours later.
Manager Aaron Boone said it was "very difficult" to tell Frazier about the minor league assignment.
Encarnacion leads the AL with 21 home runs, so he certainly will provide a boost to the AL East-leading Yankees.
Minutes after he was optioned back to Triple-A on Sunday afternoon, New York Yankees outfielder Clint Frazier admitted he was "pretty surprised" to find out he was being sent down. After signing a three-year, $60 million deal, Encarnacion is under contract through 2020.
The 36-year-old still is owed over $15 million, but The Athletic's Ken Rosenthal reported both NY and Seattle will split the money.
There had been speculation the Yankees would be in the market for a top pitcher as the July 31 approached, but the Bronx Bombers always have room for big boppers.
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Stanton is expected to be activated Tuesday when the Yankees host division foe Tampa Bay in the second game of an important three-game series.
Encarnacion was not on the field with the Mariners as they warmed up for their game at Oakland. Then, 19, was previously traded from the Mariners to the Yankees in November of 2017.
Encarnación, who is a first baseman and designated hitter, would boost what is already a power-packed lineup.
The Yankees have been one of baseball's best teams despite a long list of injuries.
Why it was surprising: The Yankees could have sent down a couple of different players, including outfielder Mike Tauchman, to add Encarnacion. Last month, they acquired first baseman/DH Kendrys Morales from Oakland, but he's struggled with NY.
The Mariners said Then, who has been pitching in extended spring training, will report to Class A Everett. Outfielder Jay Bruce was traded to the Phillies, along with more than $18.5 million of what was owed to him, on June 2 for a low-level minor-leaguer.