Alcaraz will now seek to end a challenging year – which he began by becoming the youngest man in history to complete the career Grand Slam at the Australian Open – with a statement victory at the ATP Finals in Turin.
The seven-time major winner will head into that event on a nine-match win streak, having triumphed in Vienna before his victory in Paris.
Only two days ago, Alcaraz stood on the brink of a quarter-final loss to Taylor Fritz but saved two match points to pull off a comeback victory.
It is the fourth time in his career that he has won a title after being match points down at a tournament.
That indomitable spirit was again on display as Alcaraz saved all six break points against him in a tense opening set against Shelton, who himself was fearless after a stellar year in which he has won four titles and reached a first major final.
Both players demonstrated admirable poise under pressure, with Shelton also seeing off two break points, but it was Alcaraz who held his nerve to take the first-set tie-break by winning four of the final five points from 4-3 down.
Shelton finally capitalised when Alcaraz opened the door in an error-strewn third service game in the second set. But, with the set in his sights as he served for a 5-3 lead, Alcaraz struck back.
The Spaniard pointed to his ear in a call for more noise from the crowd after edging a breathless exchange to hold for a 5-4 advantage, his confidence clear as he danced to the music inside the stadium during the change of ends.
It was not long before Alcaraz raised his arms in celebration after ending Shelton’s resistance on a second match point, embracing his team courtside after closing out the title by winning five of the final six games.
It was a ninth ATP Masters 1000 crown overall for Alcaraz, and left Canada and Shanghai as the only titles he requires to emulate Novak Djokovic and Jannik Sinner as just the third man to win all nine tournaments at that level.