The Royals traded Guillen and cash to the National League West contenders for a player to be named on Friday morning. Guillen had been designated for assignment by the Royals on Aug. 5.
The deal puts Guillen with a contender. Heading into Friday night's game against San Diego, the Giants trailed the Padres by just 2 1/2 games. The Giants also lead the NL Wild Card race.
"This kind of materialized last night," said Royals general manager Dayton Moore.
During the July 31 Trade Deadline period for non-waiver deals, the Giants often were mentioned as a possible landing place for Guillen.
In the player-to-be-named scenario, the Royals were given a list of possible candidates for the deal so that they can be scouted and evaluated with the final selection made by Oct. 15. Excluded are players on the Giants' 25-man roster, so the deal is likely to involve a Minor League prospect.
Guillen was leading the Royals with 16 home runs and 62 RBIs with a .255 average when he was dropped after three days of a West Coast trip. The move was made after the Royals could not reach a deal for the outfielder and designated hitter prior to the Trade Deadline.
At 34, Guillen did not fit into the Royals' future, which centers on younger players and, in any event, he was eligible to head for the free-agent market after the season.
The cash sent to the Giants will pay a substantial portion of the approximately $3.5 million remaining on Guillen's $12 million contract for this season. He's in the final season of a three-year, $36 million contract.
Moore said only that it was a "considerable amount."
In San Francisco, Giants manager Bruce Bochy said Guillen will become the regular right fielder when he joins the Giants. He was not available for Friday night's game against San Diego.
"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.
The outfielder whose playing time would most directly be affected by Guillen's arrival is Nate Schierholtz, but he 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.
"Jose's going to get a chance to be in a pennant race, contribute, help a team win and, hopefully, compete for a championship," Moore said.
The right-handed-hitting Guillen is a career .270 hitter with a .762 OPS and 211 home runs in 14 seasons in the big leagues. He spent 84 games as a designated hitter this season while also making 21 starts in right field.
The Royals' DH job is now being split between first basemen Billy Butler and Kila Ka'aihue. With David DeJesus injured and Guillen, Scott Podsednik and Rick Ankiel all traded, the current outfield includes Mitch Maier or Willie Bloomquist in right field, Gregor Blanco in center and Alex Gordon in left.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.