"I feel great after winning my first title," the 22-year-old Ukrainian said. "I hope more victories are to come."
Dolgopolov troubled Cilic with drop shots. He broke in the opening game of the match and easily held serve during the set.
Cilic, seeded No. 4, broke Dolgopolov decisively to lead 5-3 in the second set. Dolgopolov raced ahead 3-0 in the last set but wasted two break points and Cilic rallied to make it 3-3. The Ukrainian broke again for a 5-3 lead and sealed the victory with a crosscourt backhand.
"It was really a tough match," Dolgopolov said. "I got nervous in the third set, but I am happy I was able to win. This is a big win for me. I am really happy with what I achieved."
Cilic was the first Croat to reach the Umag final since Goran Prpic defeated compatriot Goran Ivanisevic in 1990.
"Dolgopolov was better tonight and deserved to win," Cilic said. "His fast rhythm caused me problems in the beginning. He seemed to be unbeatable the way he played. I adjusted in the second set and managed to change that."
"It was a pity how I started in the third set," he added. "I was too attacking and gave him a few points too many."