Seth Smith went 3-for-5 with an RBI and two runs scored to back Sonny Gray (5-3), who yielded three hits and three walks to go with eight strikeouts in five innings.
The American League West champion A's (96-66) will next play Friday, when they host Detroit in the start of the AL Division Series.
Mariners starter Erasmo Ramirez (5-3) was roughed up for four runs on three hits in 1 1/3 innings.
Seattle finished the season with a 71-91 mark.
Swim Week Miami Beach 2014 [PHOTOS]