Favored at 10-1, Fascinating Rock left it late but surged ahead in the final furlong to secure victory by one and a quarter length.
Found held on to second place, half a length ahead of pre-race favorite and Irish Derby winner, Jack Hobbs.
The win also makes jockey Pat Smullen the most winningmost British rider in Champions Day history.
"I think he has been an underestimated horse," Smullen later said of his horse in comments carried by the Sporting Life website.
"The ground and everything else was right for him today. They went fast, there was a good ease in the ground and 10 furlongs is his right trip."
"I wanted to be closer than I was, but they went so fast so I sat back and decided to take my chances."
Fascinating Rock's delighted trainer, Dermot Weld, agreed with Smullen's assessment.
"This was the race I thought would suit him best," Weld said. "Usually the ground is on the slow side. He loves it soft and he's a very good horse in the autumn.
"He was very good at Leopardstown when he won by six lengths and I think he'll make a lovely five-year-old.
"Hopefully he'll be back here next year to defend his title. I'll talk to the owner and see what he thinks."
Earlier in the day, pre-race favorite Solo triumphed in the Queen Elizabeth II stakes.
The French trained winner guided by jockey Maxine Guyon claimed a three-quarter length victory ahead of outsiders Belardo in second and Gabrial in third.