The six-year-old, trained by Jerome Reynier, came with a wet sail in the final furlong under Stephane Pasquier to claim the valuable first prize.
It did not look likely for most of the 10-furlong contest as Rustang set up a 12-length lead at one stage.
But Allan Smith’s four-year-old, ridden by Lee Newman, was sending out distress signals early in the straight – allowing the pack to close in quickly.
They needed to, with the winning line looming, and it was Royal Julius who got up close home to get the verdict by three-quarters of a length from Turgenev.
“He likes a right-handed horse and he likes good ground,” Reynier told Racing TV.
“He needs pace in his races – and we knew with a front-runner, we would be able to wait at the back of the pack.
“When I took the 11 number for the draw I wasn’t worried, because I like him to come on the outside without having any problem with traffic.
“That was the plan, and Stephane rode a really brilliant race. He didn’t know the horse, -he hadn’t ridden him before. I’m very happy, because this was his target for the end of the year.
“We can start next year with a lot of ambition – either in Doha to take our revenge with French King in the Emir Trophy, or maybe in Saudi Arabia.”