Hitting second in the order for the Rays, Upton hit the first pitch he saw in the first inning from Blue Jays starter Kyle Drabek and appeared to fly out to center fielder Colby Rasmus.
Instead, the ball dropped in front of the shocked Rasmus and landed on Tropicana Field's artificial surface.
"I just saw it off the bat and I turned to run because he took a big swing on it," Rasmus said. "I thought he hit it to where it was going to be close to the wall so I just turned and ran and when I looked back, I had already lost it."
Upton pulled into second base and immediately began to point up at the catwalks.
"I know the rules," said Upton, who recognized a Tropicana Field home run right away. "I kind of tossed my bat after I hit it because I figured it was an out. I came around first, I looked up and the ball just kind of disappeared and I'd been following it in the air. I knew right then that it hit the catwalk. When I got to second I knew it was a home run."
Drabek had a pretty good idea that the call would go against him.
"I looked back there and I see Rasmus kind of camped under it and then the ball drops 30 feet away from him," Drabek said. "So, I was a little confused at that but from what I know I'm pretty sure that's the rule, if it hits that catwalk, it's a home run."
Upton's discussion soon prompted Rays manager Joe Maddon to leave the dugout to ask the umpires to review the video to see if Upton's claim had legs.
"I didn't know what it hit, I just saw the ball drop straight down," Maddon said. "I didn't run right out immediately, because I was trying to evaluate what I saw. B.J. was giving me the motion. And I'm watching their center fielder, and he was camped, and that ball came straight down. Obviously it hit something."
Crew chief Joe West and his umpire crew then retreated through the visiting dugout to review the play.
After a 4-minute 57-second delay, the umpires returned to the field to announce that Upton's shot had hit one of the catwalks and therefore was a home run.
Upton then finished his trot around the bases for his third home run of the season.
Tropicana Field ground rules state that a batted ball striking either of the lower two catwalks, lights or suspended objects in fair territory is a home run. West judged that Upton's ball had hit a suspended object beyond the B-ring catwalk, thereby making the ball a home run.
The reversal was the first going the Rays' way since Evan Longoria homered on June 26 at Houston. Longoria's home run had originally been called a single and he was thrown out at second base. The only other time a Rays player had a play reviewed and overturned to be a home run at Tropicana Field came Sept. 19, 2008, when Carlos Pena hit a ball against Boof Bonser of the Twins. Initially the hit was ruled a double because of fan interference.
Blue Jays manager John Farrell had no choice but to accept the call.
"To the naked eye, it gets up into such a light-colored roof, whether or not it hit that third cat walk, I'll tell you one thing, if you hit the third cat walk in this place in center field, that is a monster blast and whether or not it did, I don't know," Farrell said. "The video that we've seen, it's hard to tell, but that was the call made."
Would Upton's ball have cleared the wall in center field if it had not hit something?
"No, no chance," said Upton, shaking his head about his home-field gift. "Hey man, playing the elements."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.