The win extended the Rays' winning streak to five games and improved their record to 13-11 for the season.
Iwamura stepped to the plate with two outs in the bottom of the eighth thinking that he might see a curveball from Clay Buchholz. Boston's starter had made a mockery of Tampa Bay's lineup to that point, holding a 1-0 lead and needing just one more out before handing the ninth inning to closer Jonathan Papelbon.
Iwamura had timed one of Buchholz's changeups in the third inning and hit a deep drive to center field, which planted the seed for the mindgame Iwamura would win.
"I took a pretty good swing against his changeup to center field, and probably [Red Sox catcher Jason] Varitek knew I had good timing on the changeup," Iwamura said. "And my first three at-bats, I had bad timing on the curveball, so I thought he would throw me a curve."
Iwamura squared up on the pitch and deposited the ball into the right-field stands for his first home run of the season.
"Hats off to [Iwamura]. I threw a pitcher's pitch, and he hit it," Buchholz said.
Rays closer Troy Percival quickly loosened up and retired the Red Sox in order in the ninth to preserve the win and earn his fifth save of the season.
Though Iwamura's clout made him the hero of Saturday night's contest, Rays pitchers kept the game close so the home run would indeed be meaningful.
Edwin Jackson started and, much to the delight of the Rays, the good one showed.
Jackson is the Rays' resident Dr. Jekyll and Mr. Hyde. One start, he can't punch a hole in the strike zone, and the next time out, he can't miss it. The constant is electric stuff, which explains the Rays' patience with the right-hander.
On Saturday night, Jackson allowed one run on five hits in seven innings.
"Basically, he was mixing up his pitches, he was a little erratic, so you kind of didn't know where the ball was going," said Red Sox first baseman Kevin Youkilis. "I think that sometimes helps out a pitcher, and he just settled down and didn't get out of control. When he walked a guy or got a hit, he settled down and didn't let it steamroll from there."
At first, the one run the Red Sox scored off Jackson looked as though it might be the difference in the game. That run crossed the plate in the fifth, when Jacoby Ellsbury's two-out single drove home Coco Crisp to break a scoreless tie. Evan Longoria dove to backhand Ellsbury's shot, but his throw could not beat the speedy Ellsbury to first. The disheartening part for Jackson came from the wild pitch he threw that allowed Crisp to advance two bases. The play turned out to be crucial, because had Crisp only made it to second, he might not have scored on Ellsbury's single.
The one run was almost enough thanks to Buchholz, who had surrendered just one hit heading into the eighth inning. Meanwhile, Rays relievers Trever Miller and Scott Dohmann combined on a scoreless eighth, feeding into the bottom half of the inning, when pinch-hitter Dioner Navarro singled with one out to set up Iwamura's homer one out later.
Buchholz "pitched real well with his curve and changeup," Iwamura said. "He changed speeds on all of his pitches, and that was very effective. ... We've been hitting real good, and we only had three hits tonight. So I'd have to say he pitched a real good game tonight."
Rays manager Joe Maddon credited both starters for their solid efforts.
"Their guy was good, and our guy was good, too," Maddon said. "I was really pleased with E.J. I really liked the look. He was pounding the strike zone. I just loved the aggressive nature. Just really a well-played baseball game."
Jackson credited the total team effort for the win.
"That's what it's going to take for us to keep winning," Jackson said. "Keep picking each other up."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.