Ka'aihue, a first baseman, was among four players called up on Monday as the Major League rosters were expanded for the final month of the season. This will be his first exposure to the Majors.
Also promoted from Triple-A Omaha were first baseman Ryan Shealy and pitchers Yasuhiko Yabuta and Devon Lowery.
They will report for Tuesday night's game against the Oakland A's at Kauffman Stadium.
Earlier, pitchers Jimmy Gobble and John Bale were recalled from their rehabilitation assignments with Omaha.
Ka'aihue, from Honolulu, Hawaii, banged a total of 37 home runs this season for two teams. He also had a cumulative 100 RBIs and a .314 batting average.
"He's had a tremendous year and he's earned the opportunity to be promoted to the Major Leagues," general manager Dayton Moore said.
For the Omaha Royals, Ka'aihue batted .316 with 11 homers and 21 RBIs in 33 games. In Double-A, he hit .314 with 26 homers and 79 RBIs in 91 games for the Northwest Arkansas Naturals.
"He's very balanced, has a consistent approach and power in his swing," Moore said.
For both clubs combined, Ka'aihue drew 104 walks while striking out just 67 times.
"That's very promising for a young power hitter to have those kinds of traits," Moore said.
Just how much playing time Ka'aihue might get is not clear, because he's one of four first basemen on the club, with Shealy, Ross Gload and Billy Butler.
Shealy had 22 homers, 65 RBIs and a .282 average in 111 games for Omaha. He had been with KC in each of the previous two seasons. He won the first-base job in the second half of the 2006 season, but was hindered by injuries last year.
Lowrey, a right-hander originally signed by the Royals in 2001, has rebounded this season from knee surgery. Pitching in relief for Omaha, he had a 2.12 ERA, a 1-1 record and five saves in 31 games. Earlier, with Northwest Arkansas, he was 1-0 with a 0.69 ERA in nine games. This is his first time in the Majors.
Yabuta started the season with the Royals after signing a two-year contract out of Japan. When he struggled with a 1-3 record and 5.46 ERA in 26 appearances, the right-hander was shipped to Omaha. In Triple-A, he had 20 outings with a 4-3 record, three saves and a 5.36 ERA.
Gobble, rehabbing from a sore back, was 1-1 with a 5.65 ERA in eight games with Omaha. Bale, out with shoulder and hand problems, was 0-3 with a 10.80 in 10 games for Omaha. Both are left-handers.
The Royals will have to make adjustments by Tuesday in their 40-man roster which does not currently include Ka'aihue, Lowery, Yabuta and Bale. They could make room by moving injured players Mark Grudzielanek, Shane Costa, Angel Sanchez and Carlos Rosa to the 60-day disabled list.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.