The Phillies announced they had acquired Sweeney on Wednesday. He will provide hitting depth for Philadelphia, which began the day two games behind first-place Atlanta in the NL East.
Seattle first activated the 37-year-old Sweeney from the disabled list. He then went on waivers before the last-place Mariners, whose season is long since lost, dealt away another of their clubhouse leaders.
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.
He's been on the disabled list twice with back pain this summer, the second time after playing 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 of 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.
Seattle entered Wednesday 21½ 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.