Justin Smoak led the Seattle offense with a solo homer, two RBI and two runs scored, helping the Mariners end a three-game losing streak. Detroit had won three in a row.
Iwakuma (13-6), who had not recorded a decision in any of his previous four starts, retired the side in order during five of the first seven innings. He worked his way out of a bases-loaded jam in the other two frames.
Smoak hit his home run in the sixth to give the Mariners a 3-0 lead and Seattle broke it open with a four-run eighth in which he had an RBI single. Franklin Gutierrez added a solo homer in the ninth.
Justin Verlander (13-12), who had ended a six-start winless skid in his previous outing, surrendered three runs on four hits in seven innings to take the loss.