The 20-year-old Italian played his first final since Antwerp on Sunday in October 2021.
"I've had a tough year so far but I kept working hard to finally lift a trophy," said the winner.
Sinner had already beaten Alcaraz less than a month ago, on the London lawn of Wimbledon at the stage of the 8th finals.
Sunday, the two men were first illustrated by their solidity in the service during a first set completed during the decisive game by Alcaraz, whose face-off was difficult to read for his opponent.
At the start of the second set, Sinner saved six break points at 0-1 before flying away to conclude 6-1 in front of an Alcaraz suddenly adrift on his first ball.
"These six break points saved may have been the key to the match. I managed to raise my level of play at that time," analyzed the Italian.
Alcaraz, who had played an extended semi-final on Saturday against the tough Giulio Zeppieri (from qualifying), during which he had twisted his right ankle in the 2nd set, seemed physically struggling against an opponent who did not hadn't had to draw so much on his reserves to make it to the final.
Blunted, Alcaraz lost 12 of the last 14 games of the game, conceding the final set 6-1.
As a consolation, the 19-year-old Spaniard, who has already won four titles this year (Rio, Miami, Barcelona and Madrid) will nevertheless climb to 4th place in the world in the next ATP ranking on Monday.
"I didn't play very well this week and Jannik deserves his victory. But I intend to find the solution to beat him next time," said Alcaraz.
Sinner became the third Italian to win at Umag after Fabio Fognini in 2016 and Marco Cecchinato in 2018.
© 2022 AFP