Guillen is a career .270 hitter and has clubbed at least 20 homers in five of the past seven seasons. He brings a much-needed power bat to a Giants offense that, despite its recent surge, has been unable to score instant runs for much of the season.
Giants manager Bruce Bochy said Guillen will become the regular right fielder when he joins the Giants.
"We didn't acquire him to be on the bench, so he'll get a lot of regular playing time," said Bochy, who added Guillen would hit in the heart of the order, but didn't yet know exactly where.
The Giants will be Guillen's 10th team in 14 Major League seasons. For much of his career, he has been tagged as a negative presence, but the Giants don't see that as an issue.
Giants starter Barry Zito, who briefly played with Guillen in 2003 while with Oakland, said he expects the vibe in the Giants clubhouse to remain where it is now.
"It's a great clubhouse, it has been all year and I think the way we're playing is a by-product," Zito said.
Guillen is in the final season of a three-year, $36 million deal signed in December 2007. The cash sent to the Giants will pay a substantial portion of Guillen's $12 million salary due him this season.
The move came the day the Giants begin a pivotal three-game series with first-place San Diego. The Padres entered Friday with a 2 1/2-game lead over the Giants in the National League West, while the Giants have a 1 1/2-game cushion in the NL Wild Card race.
This is the second time Guillen has been acquired by a team in a playoff push. In 2003, he was traded from Cincinnati to Oakland, which went on to win the American League West.
Because Guillen wasn't available for Friday's game, a corresponding move has not yet been made.
The outfielder whose playing time would most directly be affected by Guillen's arrival is Nate Schierholtz, but the 26-year-old is out of options. Tuesday's acquisition of infielder Mike Fontenot from the Cubs would allow the Giants to option infielder Emmanuel Burriss to Triple-A Fresno and still have an experienced utility infielder off the bench.