Kansas City's elimination from the post-season chase left Tampa Bay, Cleveland and Texas to fight it out for the two American League wild card berths. The Royals will miss the playoffs despite posting their first winning record in 10 years.
Iwakuma (14-6) allowed four hits and walked just one while throwing 101 pitches. Charlie Furbush completed the shutout in a non-save situation.
The game was scoreless into the fifth, but Mike Zunino put Seattle in front during that inning with the first of his two solo homer and Kyle Seager added a two-run double later in the frame.
Zunino and Michael Saunders hit home runs in the eighth.
Ervin Santana (9-10) allowed four runs on five hits in six innings to take the loss.