Brett holds the Royals franchise record with 3,154 hits from 1973-93 and led the American League in hitting in 1976, '80 and '90. In 1980, he had a late-season flirtation with hitting .400, ending the year hitting .390.
He was a 13-time all-star and was inducted into the Baseball Hall of Fame in 1999 in a record vote.
After he retired as a player, Brett served as the Royals' vice president of baseball operations and helped coach during spring training. Thursday's appointment is the first time he'll work with the team in-season.
"I'm thankful that this organization has one of the greatest hitters and more importantly one of the greatest competitors our game has ever seen in George Brett and he has accepted our offer to join the coaching staff on an interim basis," Royals General Manager Dayton Moore said in a statement.
Kansas City has lost its last eight games, scoring just 16 runs in the process going into Thursday's game in St. Louis. Only two American League teams have scored fewer than the Royals' 1,999 runs this season. Kansas City is 21-29 and in last place in the AL Central.
Brett replaced hitting coach Jack Maloof and assistant hitting coach Andre David, who were both reassigned in the Royals' minor-league system.