Saunders (3-4) bounced back from a poor performance in a 10-2 May 5 loss to Toronto, holding the Athletics to five hits while striking out six. The left-hander issued three walks.
Morales connected for his homer in the first inning and the Mariners never trailed in taking the rubber match of their three-game series.
Jason Bay added two hits in Seattle's win, including a bases-empty shot.
A's starter Tommy Milone (3-5) surrendered five runs on six hits over five full innings.
Luke Montz drove in Oakland's only run.