They obtained left-handed pitcher Jorge De La Rosa from the Brewers in exchange for infielder Tony Graffanino.
It was the third trade in two days for Royals general manager Dayton Moore.
De La Rosa, 25, has been on the disabled list since June 10 with a blister on his left ring finger. Before going on the DL, he was 2-2 with an 8.60 ERA in 18 games for the Brewers, which included three starting assignments.
This is the second straight season that Graffanino has been traded by the Royals close to the trading deadline. On July 19, 2005, they dealt him to the Red Sox for outfielder Chip Ambres and pitcher Juan Cedeno.
Graffanino was batting .268 for the Royals with five homers and 32 RBIs in 69 games. He rejoined the Royals last March 28, when he was claimed off waivers from the Red Sox.
De La Rosa has been pitching on an injury rehabilitation assignment for Double-A Huntsville since June 29. The Royals are his fourth professional organization.
He has been with Arizona (twice), Boston and Milwaukee.
Moore traded closer Mike MacDougal on Monday to the White Sox, and dealt middle reliever Elmer Dessens to the Dodgers earlier Tuesday.
De La Rosa, according to Moore, would be ready to come off the DL soon. He pitched for Huntsville on Monday night, and had nine strikeouts and no walks in five innings.
"We were able to get a quality, above-average left-handed arm," Moore said. "He's a guy who's bounced around a little bit, but is still young."
For the Red Sox's Double-A Portland club in 2003, De La Rosa had a 2.80 ERA and a 6-3 record and was named to the Futures Game. He made his Major League debut the next year.
"He has three above-average and plus pitches with a lot of movement and a lot of electricity," Moore said. "Sometimes when you have that, it's difficult to command."
De La Rosa had 38 walks and 42 strikeouts in his 42 1/3 innings for the Brewers last season.
A product of Monterrey, Mexico, he has a 4-7 record in 61 Major League games.
Graffanino's replacement on the roster was not immediately announced.
Any more trades in the immediate future?
"We're on hold right now," Moore said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.