Suppan, 23, was 1-7 with a 6.68 earned-run average in 13 starts for the Diamondbacks. He has pitched in triple-A Tucson since July 3, with a 4-3 record and 3.63 ERA.
Suppan was the player to be named in a deal last month that involved the Royals sending Jermaine Allensworth to the Mets and the Diamondbacks acquiring Bernard Gilkey from the Mets.
Herk Robinson, the Royals' general manager, said the team has kept track of Suppan since he joined the Red Sox as a rookie in 1995.
"We've always liked him," Robinson said.
Bob Bishop, a Royal scout who watched Suppan pitch in high school, went to see him play Monday in Las Vegas in a Pacific Coast League game.
Suppan is expected to be given a chance to earn a spot in the Royals' starting rotation next season.
The Royals have enjoyed success with San Fernando Valley pitching products.
Bret Saberhagen (Cleveland) won two Cy Young Awards in Kansas City. Kevin Appier (Antelope Valley High) has been the Royals' ace. Mike Magnante (Burroughs) did well with the team, and former Royal Mark Gubicza is a longtime Northridge resident.