Italian Sinner brought the house down by winning the championship match that tennis fans wanted to see, adding the prestigious year-ending tournament to the Australian Open and a landmark triumph at Wimbledon this season.
The 24-year-old also bounced back from a three-month ban, which chopped out a large chunk of his season despite the World Anti-Doping Agency accepting that the Italian was accidentally contaminated with the banned substance clostebol last year.
"It has been a really long week... it was a brilliant atmosphere here, it was almost like being at a football match," said Sinner on court to loud cheers.
"I'm really happy to have given something positive to all of you."
Sinner has now won 31 straight matches on indoor hard courts, a run which stretches back to the 2023 championship match at Turin's Inalpi Arena, which he lost to Novak Djokovic.
Four more indoor wins would draw him level with Djokovic's tally set between 2012 and 2015 - the second-highest in the Open era but some way behind John McEnroe's record of 47.
He hasn't dropped a set at the Finals since losing that final to the Serb two years ago, and he was imperious over the week in northern Italy.
Alcaraz, unlike Sinner, now heads into the Davis Cup after a remarkable individual season in which he has won eight tournaments in total and taken his Grand Slam tally to six.
Spaniard Alcaraz had the edge over Sinner over the course of the season, winning four of their six match-ups, which all decided top-level tournaments, and pipping his rival to the year-end world number one spot.
"I'm really, really happy with the level that I played today, with the performance," said Alcaraz.
"I mean, I just played against someone who hasn't lost a match indoors for two years now, so that means you know how great a player you are," Alcaraz told Sinner.
While both Sinner and Alcaraz breezed into the final, the championship game was an attritional affair, with each player rock-solid on serve until an enthralling tie-break at the end of the first set.
Sinner took the lead thanks to a brilliant lob which set up a set point, and he made no mistake with a missile of a serve which Alcaraz could only limply send wide.
But Sinner immediately handed Alcaraz the advantage in the second set with two double-faults, which helped hand his opponent a break of serve at the start of the frame.
Sinner hadn't dropped a service game in the whole tournament up to that point, but he broke back in game six to put the crowd on their feet.
And Sinner collapsed to the ground in joy when Alcaraz sent a backhand wide on the first championship point, before heading into the stands to share his joy with his family amid the roars of the crowd.