The chestnut colt became the second undefeated horse to sweep the Kentucky Derby, Preakness and Belmont, improving to 6-0 in a racing career that began on February 18.
Bob Baffert became the second trainer to win the Triple Crown twice.
Last year, Baffert won four races with Mor Spirit (Metropolitan Handicap), Abel Tasman (Acorn), American Anthem (Woody Stephens) and West Coast (Easy Goer).
"It's been the longest, quickest journey", Baffert said Sunday. "It was meant to be".
He didn't flinch when greeted by 90,327 roaring fans as he walked onto the track.
Jockey Mike Smith, 52, is the second-oldest Kentucky Derby victor behind Bill Shoemaker. "This achievement elevates Justify into the pantheon of America's all-time greatest thoroughbreds".
Justify will be trying to become thoroughbred racing's 13th Triple Crown victor.
Scat Daddy's bloodlines include Canadian-breds Nijinsky (an English Triple Crown champion), Northern Dancer and Storm Bird.
For Justify, being one of the finest athletes of our time will have to do. Justify, who became the 13th Triple Crown victor, was purchased as a yearling for $500,000. Slew also was a wire-to-wire Belmont victor.
Justify's human handlers also made history.
American Pharoah was the last horse to win the Triple Crown, capturing horse-racing's storied trifecta in 2015. James "Sunny Jim" Fitzsimmons guided Gallant Fox in 1930 and Omaha in 1935.
Justify's jockey is Mike Smith, and at 52 years old he is still considered a big-money rider. He celebrated by grabbing white carnations from the winner's blanket and tossing them in the air.
"It baffled me", Mike Repole, who owns fellow competitors Vino Rosso and Noble Indy, said, per David Grening of Daily Racing Form.
Concern over Justify's No. 1 position was eliminated right away, as Smith recalled the start.
With rain and mud a strong possibility again, in NY on Saturday, the question was which Justify would we see?
Justify made history on Saturday by winning the Triple Crown, which reportedly increased the value of his breeding rights, according to ESPN.
Chris Mara has won two Super Bowl championships with the New York Giants, and now he can add the Triple Crown to his resume. He was purchased as a yearling for $500,000.
Some had wondered whether the colt might show signs of fatigue following a busy schedule but that was not the case as he broke from his number one post position and was never seriously challenged by any of the other nine horses.
On a fast, dry track, Justify was just as good in his third race in five weeks at his third different track. The race was won by a half-length, with Justify finishing the 1 3/16-mile race in 1:55.93. But jockeys say Smith's age is actually an advantage. "He's sent from heaven".
Jockey Jose Ortiz rode the rail through the final turn of the 1 1/2-mile test, angled out, and loomed behind Justify, ready to break plenty of hearts.
A win for Justify would be Bob Baffert's second as a trainer. They didn't do it. But he shot out with a fury, took a 11/2-length lead at the quarter pole and never let a challenger get closer than that.
"He ran a large number of races in a short amount of time with no 2-year-old foundation", said Chad Brown, who trains Gronkowski.
"I've wanted to see that horse's name up there because we knew he was brilliant from day one". The colt named for New England Patriots tight end Rob Gronkowski brings a four-race winning streak into his US debut.
The season-ending Breeders' Cup world championships are in Louisville on November 2-3.