The Wimbledon champion was simply too strong for the Spaniard as he cruised to a 6-2 6-4 win in 73 minutes in Riyadh.
World No.2 Sinner collected the $US6 million ($A9.27m) winner's prize at the lucrative six-player event, having also bested Stefanos Tsitsipas and Novak Djokovic on his way to the trophy.
As the Six Kings Slam is an exhibition tournament, rather than an officially sanctioned ATP tournament, none of the results count toward the players' tour rankings and tour head to head records..
The Italian took an early grip on the first set when he broke in the opening game before holding serve in the second, and although the US Open champion got himself on the scoreboard in the next, the respite proved temporary.
The Italian held to love and then forced two break points in the fifth game, and when Alcaraz found the net with his backhand, Sinner led 4-1, an advantage he converted to take the opening set 6-2 in emphatic style.
Alcaraz re-grouped at the start of the second, holding comfortably in the opening game and, after Sinner had levelled with the minimum of fuss, repeated the feat to lead 2-1.
Serving at 2-2, the Spaniard found himself 15-40 down and had to save four break points before eventually emerging victorious, but he was not so fortunate in his next service game as his opponent broke once again to lead 4-3.
The big-serving Italian confidently eased his way to within one game of victory and although Alcaraz made it 4-5, Sinner powered his way across the finishing line at the first time of asking.
Earlier, 24-times grand slam winner Novak Djokovic had retired from his third place play-off clash with America's Taylor Fritz.
The 38-year-old shook hands after Fritz, who had lost his previous 11 clashes with the Serbian, had taken the first set on a tie-break after a gruelling 75 minutes.
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