The Phillies expect him to arrive on Thursday, before the final game of their three-game series with the Marlins at Sun Life Stadium.
Sweeney, a five-time American League All-Star, came into the season with a career .298 batting average over 16 seasons. He was batting .263 with six home runs and 18 RBIs over 30 games for the Mariners this season, including three starts at first base.
Sweeney has hit at least 20 home runs in six seasons, including 29 in back-to-back seasons (2000-01).
"He's always been able to hit the ball," said Phillies outfielder Raul Ibanez, who has known Sweeney since they were on the same team in Class A.
"He has some power, too," Ibanez added. "He's played in some big parks, but he's always hit the ball hard."
Sweeney's hitting pedigree combined with the fact he has made 570 career starts at first base prompted the Phillies to make the move.
According to Ibanez, they also are getting a special person.
"He's a quality, upbeat human being," Ibanez said. "He's one of the nicest teammates I've ever had and one of the nicest people I've ever met. He brings positive energy everywhere he goes every single day."
Ibanez said Sweeney will lead by example but "can pull somebody aside" for counsel. And he said he is capable of "getting in someone's face" should it be necessary.
Originally selected by the Royals in the 10th round of the 1991 First-Year Player Draft, Sweeney had 213 home runs and 901 RBIs in 1,428 games for Kansas City (1995-2007), Oakland (2008) and Seattle (2009-10).
Notably, he has batted .328 in his career with runners in scoring position. And in 27 career games against National League East teams, he has hit .320.
Now the Phillies just hope to keep him healthy.
Sweeney has been on the disabled list twice this season with back inflammation/spasms. Most recently, he was on a rehab assignment with Triple-A Tacoma. In 12 games there, he hit .366 with two home runs and nine RBIs.
The Phillies must make a move on the 25-man roster to accommodate Sweeney when he reports.