Third in the race 12 months ago, trainer Jean-Claude Rouget had stated that only one day mattered for the four-year-old this year and he proved trained to perfection.
For John Gosden's Enable, chasing history with a third victory in the race, she was bogged down in the heavy ground and suffered interference when the pace quickened.
In contrast, Sottsass picked up smartly and stayed on well to beat In Swoop, with Persian King hanging on for third having made the running, just ahead of Gold Trip. Enable's triple Gold Cup-winning stablemate Stradivarius was seventh.
As a result of the interference, a stewards' inquiry was immediately called - but following a short deliberation, the placings remained unaltered.
While Enable did not manage to win a third Arc, she will still go down in history as one of the greatest racemares to have graced the sport.
As well as her two wins in the Arc, she won the King George VI And Queen Elizabeth Stakes at Ascot three times, the English and Irish Oaks and at the Breeders' Cup.
Sottsass won the French Derby last season, but had failed to set the world alight this year. However, Rouget had been making increasingly confident noises in recent days and his colt had been well-backed.
Cristian Demuro enjoyed a perfect run through the early stages, tracking Persian King with Frankie Dettori just a length behind the eventual winner.
When the pace quickened in the straight Dettori had Enable in the perfect position to strike, with Stradivarius on her outside, but she when she needed to quicken, the heavy ground meant she was unable to use her raking stride and she was beginning to send out distress signals when squeezed up.
Enable is Juddmonte's most successful racehorse, a fourth generation homebred, a history-making three-times King George winner, with the most successful record in 100 years of the Prix de l'Arc de Triomphe. We ❤ her more than ever #QPAT pic.twitter.com/7nO11ofEIr— Juddmonte (@JuddmonteFarms) October 4, 2020
In Swoop, the German Derby winner, was predictably strong at the finish, but Sottsass was always doing enough and held on by around half a length.
Rouget said: "Just after the race last year we thought he was a horse made for this race.
"Between the Ganay (in June) it was a long time. When we ran in Deauville (in August) he was a bit fat and and Skalleti is a very good horse. He is a Group One horse on soft ground, but we had to run in that race instead of going to York (Juddmonte International).
"The choice to go to Leopardstown (Irish Champion Stakes) was tough, too, and not the Foy. We chose to run him over a shorter distance to give him speed. I think that was a good choice.
"All was made to have the horse 100 per cent today. The result is there.
"The fact the (Aidan) O'Brien horses were not there was easier to understand how the pace will be. I was not surprised to see Persian King in front, because for him it was the best way because he did not pull - he ran a magnificent race at this distance because it was not his trip or his ground.
"We had a good draw to stay behind him and our horse stays the distance better than him.
"John Gosden is a good trainer and if I was beaten I wanted it to be by Enable because she is a super filly, but there is only one winner in the race."
He added: "Sottsass is owned in association with Coolmore. I do not know if he will run again, but it is not the question for today.
"Whether the horse retires is not my decision."
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