Showers are possible for the 1 1/2 mile (2400m) race on the Belmont Park dirt in Elmont, New York.
The Belmont Stakes, to be held for the 150th time, is the longest of the triple crown races.
Justify won the Kentucky Derby by 2 1/2 lengths and triumphed narrowly at the Preakness Stakes -- both races run in sloppy conditions.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"I think he's got to run better than he did in the Preakness but we know going in he's got the most talent," NBC horse racing analyst Randy Moss said on Friday.
Justify had never raced until February.
Should he win, he will be only the second undefeated horse to complete the triple crown, joining Seattle Slew (1977).
Ridden by two-times Belmont Stakes winning jockey Mike Smith, Justify has drawn the inside barrier.
After wearing mainly white silks in the previous two triple crown races, Smith on Saturday will don the colours -- red with yellow stars -- of the China Horse Club, part owner of the three-year-old colt.
Hofburg, a 9/2 shot ridden by Irad Ortiz Jr., is rated by punters as the most likely to beat Justify. He has not run since a seventh placing at the Kentucky Derby five weeks ago.
Bravazo and Vino Rosso, both 8/1, are also considered legitimate threats.
Luis Saez-ridden Bravazo nearly ran down a fading Justify in the final yards of the Preakness before finishing second, a half-length behind.
Vino Rosso, ridden by John Velazquez, had a bad start at the Kentucky Derby but battled on to finish ninth after starting from a wide barrier.
Latest Belmont Stakes betting odds from Morning Line:
Justify 4/5
Hofburg 9/2
Bravazo 8/1
Vino Rosso 8/1
Gronkowski 12/1
Tenfold 12/1
Blended Citizen 15/1
Noble Indy 30/1
Restoring Hope 30/1
Free Drop Billy 30/1
(Reporting by Andrew Both in Cary, North Carolina; Editing by Toby Davis)
