Kansas City dropped their fourth straight game on Sunday 2-1 to the Mariners to start their homestand with a sweep.
Manager Ned Yost unveiled a shuffled Royals (39-36) lineup in lieu of Danny Valencia's return from the disabled list. The team optioned Louis Coleman back to Triple-A Omaha to make room.
Justin Maxwell, one part of the jumbled batting order, scored the first run on a sacrifice fly in the second.
Yordano Ventura (5-6) only allowed two hits through the first four innings while nursing the slim lead. Former Royal Willie Bloomquist then doubled to foul territory in right, scoring Mark Zunino for the tying run in the fifth.
Zunino put Seattle (40-36) on top with his ninth home run of the year in the seventh.
Ventura gave up six hits and struck out six over his seven innings.
Mariners starter Roenis Elias pitched 6 2/3, striking out five Royals while allowing five hits.