Seattle matched its season high with its fifth straight win, won its ninth in a row over the A's and moved within two games of the idle Yankees for the second AL wild-card spot with nine games remaining. Oakland lost its season-worst sixth straight home game and fell four games back of New York.
Bassitt, a 32-year-old right-hander, was struck by a 100 mph line drive off the bat of Brian Goodwin of the Chicago White Sox on Aug. 17 and had surgery seven days later on three fractures.
He took the field to a standing ovation from the crowd of 4,966 and allowed one hit in three scoreless innings - J.P. Crawford's leadoff single - struck out four and walked one.
"I am very blessed and thankful for the circle that I have made in baseball," Bassitt said. "When things happen like that, you really find out who's with you and I'm beyond blessed to have the people that I have with me."
Oakland led 3-0 when Abraham Toro hit an RBI double in the fourth. Sean Murphy boosted the lead with a home run in the bottom half, but Cal Raleigh cut the deficit to 4-3 with a two-run homer in the fifth.
Haniger's 35th homer tied the score in the sixth, and Luis Torrens followed with a go-ahead, two-run homer off Jake Diekman (3-3).
"We're doing a lot of things right, I think, and you need to," Mariners manager Scott Servais said. "We've still got a lot to play for here with nine games to go."
Joe Smith (3-4) pitched a scoreless fifth inning. Matt Olson hit his 38th home run, a two-out drive in the ninth off drive off Paul Sewald, who retired Jed Lowrie on a popout for his 10th save in 14 chances.
"I'm proud of this group, even though we've absolutely sucked the last month," Bassitt said. "We haven't done a good enough job to win games."
Oakland built a 3-0 lead on a second-inning sacrifice fly by Elvis Andrus and third-inning RBI hits by Yan Gomes and Mark Canha off Yusei Kikuchi, who gave up three runs, three hits and four walks in three innings.
Mariners right-hander Casey Sadler pitched two shutout innings out of the bullpen to extend his scoreless streak to 24 straight appearances, one shy of Shigetoshi Hasegawa's franchise record, set in 2003. Sadler hasn't allowed a run in his last 22 2/3 innings.
Athletics: Lowrie (hand) doubled as a pinch hitter in the seventh inning. The switch-hitter is currently only available to bat from the left side.
The A's placed LHP Sam Moll on the paternity list and released C Aramis Garcia after designating him for assignment on Monday.
Mariners: RHP Logan Gilbert (6-5, 4.74 ERA) opens a three-game series against the Los Angeles Angels on Friday night in Anaheim. Gilbert is 2-0 with a 2.61 ERA in two previous starts against the Angels.
Athletics: RHP Frankie Montas (12-9, 3.57 ERA) takes the hill on Friday night as Oakland welcomes the first-place Houston Astros for a three-game series. Montas has won three straight decisions since his last loss on Aug. 16.
Mariners use their power to get past Athletics
Cal Raleigh, Mitch Haniger and Luis Torrens go yard in the Mariners' win over the Athletics.