The fourth-ranked Austrian, who didn't drop a set all week in the last clay-court event of the season, called it "a big childhood dream of mine" to win the high-altitude tournament in the Austrian Alps.
"It is absolutely incredible, one of my biggest dreams since I play tennis," said Thiem, adding he always wanted to win the event since visiting the venue as a 6-year-old.
Thiem had reached the final in Kitzbuehel once before, in 2014, but lost to David Goffin.
"There is no better thing for an athlete than having this kind of success in front of your home crowd," said the 25-year-old Austrian, who joined Novak Djokovic and Roger Federer as the only players who have won three tournaments this season.
Thiem ended the successful run of Ramos-Vinolas, who reached the semifinals in Bastad, Sweden, two weeks ago before winning the second title of his career in Gstaad, Switzerland.
The 69th-ranked Spaniard led 5-4 in the opening set when the match was suspended because of rain, but Thiem dominated after play resumed and conceded just two more games.
He closed out the victory on his first match point when Ramos-Vinolas hit a return long.
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