Alcaraz, the No.1 seed, defeated Giulio Zeppieri over three sets in Sunday's (AEST) semi-final , securing his 42nd win of the year.
That is the joint-most victories in 2022 on the ATP Tour alongside Stefanos Tsitsipas, and extends his record in Umag to 8-0.
The 7-5 4-6 6-3 win sees Alcaraz reach an ATP Tour-leading sixth final in 2022 and will lift him to No.4 in the world rankings ahead of Tsitsipas, a career high for the 19-year-old.
"It's amazing to be in a final here again. I enjoy playing here in Umag and I'm really happy with the win today," Alcaraz said.
Alcaraz will bring up his first century of ATP matches when he tackles Italian Sinner.
"It would be amazing in my 100th match to get the win, and of course it is amazing to be number four on Monday," Alcaraz said. "It's something great for me, but right now I am just focused on the final."
Sinner, who beat Alcaraz at Wimbledon to set up a quarter-final against Novak Djokovic, saw off compatriot Franco Agamenone 6-1 6-3 in the other semi-final in Croatia.
The 20-year-old Sinner is looking for his first title of 2022, having won at Antwerp, Sofia, Washington and Melbourne last year.
He has reached the quarter-finals in two of the three grand slams played so far this year.