Perfectly ridden by Peru-born jockey Jose Valdivia, Ruler On Ice defied his generous odds of 24-1 to outstay his more favoured rivals and win the one-and-a-half mile classic and claim the third jewel of American racing's Triple Crown after missing the first two legs.
"With a half-mile remaining, I still felt like I had a lot of horse beneath me," Valdivia said.
"Turning for home, everything started going in slow motion. At the 16th pole, I thought 'this is the true test of a champion.'"
Stay Thirsty, another long shot, finished second by three quarters of a length after hugging the rail down the stretch while Brilliant Speed grabbed third, one-and-a-half lengths behind the runner-up, after a late charge down the centre of a sloppy Belmont Park track.
Animal Kingdom, which started favourite after winning last month's Kentucky Derby and finishing a close second in the Preakness Stakes, finished sixth after the colt blew his chances when he was bumped as the 12 runners charged out of the gates. John Velazquez was left clinging desperately to the reins after his foot was knocked out of the stirrups. He was a distant last for most of the journey until he managed to get his foot back in and set off on a wide run but was unable to catch the leaders.
"It was unbelievable. They came over on me and clipped heels and I almost came off. I had a horrible trip," Velazquez said.
"I was just hoping to get a piece of it and that's why I rode him that way after the start. No way he was going to make up that much ground. He's still a great horse."
As with the two previous legs, the Preakness winner Shackleford jumped to the front and tried to lead all the way. He was still in front when they turned for home but faded to finish fifth.
"He did everything he was supposed to do and didn't hang on," Shackleford's trainer Dale Romans said.
Despite recording two wins and three minor placings from his six previous career starts, Ruler On Ice, who paid $51.50 (32) for a $2 (1.23) win bet, was still regarded as an outsider after failing to get a start in last month's Kentucky Derby and opting to skip the Preakness.
His trainer Kelly Breen said the horse had a habit of losing concentration during his races so they put blinkers on him for the first time, confident that if his manners improved he would run the distance because of his breeding.
He made a good start from an inside post position and was second behind Shackleford for most of the way before surging to the front in the final straight and hanging on to collect the first prize of $600,000 (370,000) and the traditional winner's garland of white carnations.
"He didn't mature as fast as we wanted to. He's already a gelding so we can't castrate him again, so we put blinkers on," said Breen, who claimed his first Triple Crown success.
"He was still goofing off and didn't break great. Last week, he broke right and it's like sometimes, the bulb just goes on. It was a perfect storm of things going right, and that's how we got here."
Hopes had been high for Aidan O'Brien's Master Of Hounds after his fifth-place finish in the Kentucky Derby behind Animal Kingdom but the Ballydoyle runner faded badly in the straight.
Ruler On Ice's victory makes it three years in a row a different horse has won each of the Triple Crown races, and next year it will be a 33-year gap since Affirmed swept the Derby, Preakness and Belmont in 1978.
The winning time for the oldest and longest race in the Triple Crown was a slow 2:30.88.