The 25-year-old has struggled mightily this season, hitting .152 with four home runs and 17 RBIs in 40 games.
"Moose," who the Royals selected No. 2 overall in the 2007 draft, has spent four seasons with the Royals, and he has 41 career home runs and 162 RBIs. He hit 20 home runs two seasons ago.
Royals manager Ned Yost said last week that he'd give Moustakas time to iron out his troubles at the plate, but he was only 3-for-16 at the plate in the six games since then.
The Royals called up utility man Jimmy Paredes to take Moustakas' place on the 25-man roster. Danny Valencia will be the team's regular third baseman.
Kansas City (23-23), which has the fewest home runs in the majors (20), is in third place in the AL Central heading into a three-game set in Anaheim that begins on Friday. SI WIRE: Astros outfielder George Springer suffers hip flexor strain in loss to Angels