Boston sent catcher Tim Federowicz and right-handers Juan Rodriguez and Stephen Fife to the Los Angeles Dodgers, who dealt OF Trayvon Robinson and OF Chih-Hsien Chiang to the Red Sox.
The Red Sox then sent those two prospects to Seattle.
"We talked with a few other clubs but in the end we thought this was the one that made sense," Mariners general manager Jack Zduriencik said.
AL East-leading Boston was in need of a starter to help shore up its rotation with Daisuke Matsuzaka out for the season and Clay Buchholz on the disabled list.
The Mariners sent Baltimore five players for Bedard before the 2008 season but he has been a bust because of injuries.
Bedard was 15-14 with a 3.31 ERA in 46 appearances with the Mariners. He missed most of the 2009 season and all of 2010 after left shoulder surgery. He was 4-7 with a 3.65 ERA in 16 starts this season but spent most of July on the disabled list (knee).
Fields, 25, the 20th player taken in the 2008 draft out of Georgia, was 2-1 with a 2.77 ERA for Double-A Jackson and 0-0 with a 6.23 ERA in nine games for Triple-A Tacoma.
"We've been talking Chiang for a while. Robinson came in a couple hours (before the deadline) as we discussed it," Zduriencik said. "He was the guy we settled on."
The Mariners have been struggling offensively — the Mariners designated Jack Cust for assignment Friday — and both 23-year-old prospects have power potential.
Robinson is hitting .293 with 26 home runs and 71 RBI for Triple-A Albuquerque.
"He'll go to Tacoma and we'll watch him there," said Zduriencik, when asked if Robinson was close to a promotion. "We'll see what happens."
Chiang is hitting .338 with 36 doubles, 18 home runs and 76 RBI in 87 games for Double-A Portland.