The Phillies announced they had acquired Sweeney a day after they put All-Star first baseman Ryan Howard on the disabled list with a sprained ankle.
Sweeney will provide hitting depth for Philadelphia, which began the day two games behind first-place Atlanta in the NL East. The Phillies are hopeful to have Howard, the 2006 National League MVP who was leading the NL with 81 RBIs when he got hurt, back in two weeks.
"He is a good hitter. I saw him a lot in the American League," Phillies manager Charlie Manuel said of Sweeney from Miami, where Philadelphia is playing the Marlins. "He's going to play some first base now since Howard is out and he'll be good off the bench, too, down the stretch. I'm looking forward to him playing for us and I like it."
Manuel said Sweeney should join the Phillies Thursday for the final game of the Marlins series.
Playing first base could become troublesome for the 37-year-old Sweeney. He has been on the DL for back issues twice this summer.
Seattle first activated Sweeney from the disabled list on Wednesday. He then went on waivers before the last-place Mariners, whose season is long since lost, dealt away another of their clubhouse leaders.
"Mike is true pro and was a contributor to our organization in many ways, both on and off the field," Seattle general manager Jack Zduriencik said. "This is a great opportunity for Mike and we are pulling for him to succeed in Philadelphia."
Sweeney, considered to be one of the most universally liked players in the game, appears to be headed to a role as a pinch-hitter in Philadelphia. He has been bothered by low back pain this season and had knee problems throughout last season.
Playing first base, even on a limited basis, exacerbated those pains and left him primarily as a part-time designated hitter in the American League.
The second time Sweeney went on the DL this summer, June 27, came after he played one game at first base. Sweeney took himself out of a game last week with Triple-A Tacoma while there on a minor-league rehabilitation stint, but felt better the next day.
He was batting .263 with six home runs and 18 RBIs in 30 games with Seattle this season.
This will be the first time in Sweeney's 16 seasons he has played in the National League. The career .298 hitter spent 1995-2007 with Kansas City and had all his All-Star selections while with the Royals. The last was in 2005.
Sweeney, who has 219 career home runs, was with Oakland in an injury-shortened 2008 and thought he may be retiring before he signed with Seattle. He made the Mariners in each of the last two seasons as a non-roster invitee to spring training.
He has hit at least 20 home runs in six seasons, including 29 in back-to-back seasons (2000-01). The former catcher has made 570 career starts at first base.
Seattle entered Wednesday 21 1/2 games out in the AL West and has lost both of its clubhouse leaders in the span of two months. Ken Griffey Jr. abruptly retired in early June, weeks after Sweeney challenged any teammate who leaked word to the media that Griffey had been asleep in the clubhouse during a game to a fight. Sweeney's challenge came during an emotional team meeting.