Morse doubled and scored in the second inning, hit his first two-run shot in the fourth off Reds starter Homer Bailey and connected for his second two-run shot in the fifth off reliever Clay Hensley. Morse is 11-for-31 in spring training and leads the team in home runs with five.
Kendrys Morales and Justin Smoak added two hits and an RBI each for Seattle, which snapped a four-game losing streak.
Bailey allowed three runs on six hits in four innings. Zack Cozart had two hits including a two-run single in the fourth inning and Chris Heisey had two hits for the Reds.
All-Star first baseman Joey Votto went 0-for-2 with a run scored before being replaced after three innings because of illness and fatigue.
Mariners left-hander Joe Saunders was roughed up for five earned runs on nine hits in 3 2-3 innings. He pitched through pain from a blister on his left foot.