The Italian has got better and better as the season has gone on, despite the doping case hanging over his head that could still lead to a suspension.
His 6-4 6-4 victory over fifth seed Taylor Fritz made it 26 wins from his last 27 matches, a run that included his second grand slam title of the year at the US Open, two Masters 1000 trophies and now the ATP Finals.
By sweeping all five of his matches en route to the trophy, Sinner earned $7.4 million (US$4.8 million), the largest winner's prize on the men's tour.
"The work will never stop. We know we still have room for improvement," Sinner said. "But it's been an incredible season and there's no better place to finish the season than here."
Sinner has become the first Italian to win the finals and he went one step further than last year, when he lost the championship match to Novak Djokovic, who withdrew this time. And he did so without dropping a set, which was last accomplished by Ivan Lendl in 1986.
"I just tried to understand whatever works best for each opponent," Sinner said. "It was a very high-level tournament from my side. At times, I couldn't play better."
Sinner faced a break point while serving for the first set but saved it with a big serve out wide that Fritz was unable to return. Then he served an ace, his 10th of the set, to close it out.
Another break by Sinner early in the second and the match was virtually over.
"He served absolutely lights out. So many lines," Fritz said. "He placed the serve great. "He did a great job of not only mixing up the second but being very aggressive with it."
Sinner ends the ATP season with eight titles and an overall record of 70-6, having won his first two grand slam titles at the Australian Open and US Open and clinched the year-end No.1 ranking.
And the year could yet have one more triumph, with Sinner set to head to Malaga, where he will attempt to help Italy successfully defend the Davis Cup title.
But Sinner also tested positive in two separate drug tests in March and a decision to clear him of wrongdoing was appealed by the World Anti-Doping Agency in September. A final ruling is expected from the Court of Arbitration for Sport early next year.
Sinner's explanation was that the banned performance-enhancer entered his system unintentionally through a massage from his physiotherapist, who had used a spray containing the steroid to treat his own cut finger.
US Open finalist Fritz will rise to a career-high No. 4 in the rankings on Monday after beating No. 2 Alexander Zverev in a third-set tiebreaker in the semi-finals on Saturday.
That will make him the highest-ranked American man since Andy Roddick was No. 4 in August 2007.
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