Fister allowed one hit in six scoreless innings in his major league debut, and he turned a one-run lead over to the bullpen.
But with Seattle on the verge of its 28th win this season in a one-run game, Ramirez jumped on a 2-2 fastball up in the strike zone from Aardsma and lined it 370 feet into the White Sox bullpen in left field.
The victory moved Chicago within two games of first-place Detroit in the AL Central and gave White Sox starter John Danks his fifth win in his last seven starts.
"These games can go either way. I think if we lost this game it could really hurt our ball club, the way we were playing early, the way we were swinging the bat," Chicago manager Ozzie Guillen said. "Seeing Danks, the way he was throwing the ball and not giving him anything, there was a little bit of frustration."
It was the first time this season the White Sox won when trailing after eight innings.
Paul Konerko nearly tied it to lead off the ninth, but was a 400-foot out when Franklin Gutierrez tracked down his drive at the wall in center. After a walk to A.J. Pierzynski and a single by Carlos Quentin, Ramirez hit his 13th homer of the season off Aardsma (3-4), who suffered his third blown save of the season.
"No loss is easy but obviously when you give it up in the ninth it stings a little bit more," said Seattle manager Don Wakamatsu, who saw his club fall 5½ games behind Boston in the AL wild card. "It hurts quite a bit."
The stunning turn made a deserving winner out of Danks, who tied his career high with eight innings pitched. He scattered seven hits, struck out eight and threw just 98 pitches.
Chicago closer Bobby Jenks pitched a perfect ninth for his 23rd save in 27 attempts. It was the first save since July 20 for Jenks, who had blown his last two save opportunities.
But Danks (10-8) was aware of the zeros being thrown up by Fister and knew he couldn't allow more than Branyan's hit.
"As the game went on I had zero room for error. I did give up the run in the fourth, but these guys picked me up tonight," Danks said. "Without a doubt I knew what Fister was doing over there and it really does push you to bear down and try not to give up any more runs."
Before the ninth, the only Chicago hits failed to leave the infield. Jim Thome had an infield single in the first, when Chicago threatened for the only time against Fister. And Gordon Beckham chopped an infield single in the eighth off reliever Mark Lowe, but was erased by a double play.
Chicago passed its hit total for the game in the ninth with the three hits off Aardsma, whose last blown save was July 8 in a 5-3 loss to Baltimore.
The meltdown ruined what appeared to be a perfect debut for Fister. He walked four batters and hit another, but kept the White Sox off the scoreboard and never found himself in major trouble. The lanky 6-foot-8 right-hander went 6-4 with a 3.81 ERA at Triple-A Tacoma, and the Mariners took a chance in calling up Fister and demoting lefty Jason Vargas.
With a lot of family in attendance, Fister left after the sixth inning to a standing ovation after striking out Thome and Konerko.
"There is always one or two pitches that determine a game a lot of times and it didn't bounce our way this time," Fister said.
- Chicago OF Jermaine Dye had his seventh and eighth assists of the year. He threw behind Branyan and caught him in a rundown on his RBI single, then nailed Branyan trying for a double in the seventh.
- Seattle C Rob Johnson missed his second straight game with a sore right elbow, but Wakamatsu intends on playing Johnson on Wednesday.
- Guillen plans on playing newly acquired OF Alex Rios in Wednesday's series finale, but wasn't sure what position he would play.