Jose Ramirez (1-0), the second of eight Seattle pitchers, retired five batters for the win.
The two teams, who entered with the worst records in the AL West, combined for 28 hits, including six doubles, one triple and three home runs.
Danny Valencia had two hits, including a grand slam for Oakland.
Seattle trailed 4-0 and 5-1 before scoring a season-high six runs while batting around in the third.
Romero, a September call-up who entered the game in the bottom of the second after Franklin Gutierrez was forced out with tightness in his right groin, had an RBI single as part of Seattle's big inning. He hit his first homer of the season in the fourth.
The A's pulled to 9-8 in the sixth before Seager hit his 22nd home run in the ninth.
Tom Wilhelmsen pitched the ninth for his ninth save.
Most of the scoring came in the first four innings after both starters were knocked out of the game.
The A's took an early lead on Valencia's third career grand slam off a 1-1 pitch from Seattle's Edgar Olmos. Olmos retired just four batters and gave up five runs in his second major league start.
Brett Lawrie's RBI double in the second extended Oakland's lead to 5-1 before Seattle roared back with a season-high six runs in the third against Aaron Brooks. The Mariners had four doubles in the inning, including back-to-back from Morrison and Brad Miller that drove in two runs apiece.
Brooks (1-2) allowed six runs in 2 1/3 innings.
Marcus Semien had a two-run triple for the A's.
Athletics: Stephen Vogt had a pinch-hit single in the seventh after being scratched from the starting lineup due to soreness in his right elbow. ... OF Coco Crisp singled as a pinch-hitter in the sixth after sitting out three games with a sore neck. ... OF Sam Fuld continues to be sidelined with back spasms. ... RHP Sean Nolin was recalled from Triple-A Nashville before the game and is scheduled to pitch the finale of this series.
Mariners: RHP Felix Hernandez (15-8) has 21 career wins against the A's but lost in his last appearance at Oakland on July 4, his first there since 2008.
Athletics: RHP Jesse Chavez (7-13) has given up at least one home run in seven consecutive starts, the longest streak by an Oakland pitcher over the past two seasons.