Kila Ka'aihue will be optioned to Triple-A Omaha, while catcher Jason Kendall will be transferred to the 60-day Disabled List to make room for Hosmer on the 40-man roster.
Hosmer, a 21-year-old first baseman, batted .439 (43-for-98) in 26 games for the Storm Chasers and was leading the Pacific Coast League in batting average, hits and on-base percentage at the time of his departure.
The slugging first baseman was riding an eight-game hitting streak in which he was batting .613 (19-for-31) with two homers, six RBIs and eight walks. His .613 batting average and .692 on-base percentage led all of professional baseball during that span.
The No. 3 pick of the 2008 Draft, Hosmer is the first high school player from his class to reach the big leagues. The left-handed swinger is expected to make his Major League debut against Gio Gonzalez of Oakland, one of the toughest southpaws in the American League.
During his time in Omaha, Hosmer batted .500 (16-for-32) with all three of his home runs against lefties.
Ka'aihue was batting .195 (16-for-82) with two homers in 23 games this season for the Royals. He has a .285 batting average with 52 home runs in parts of three seasons in Omaha.