Chilean Jarry, the world number 54, beat Dimitrov 7-6 (7/1), 6-1 at the Parc des Eaux-Vives to claim only the third singles title of his career.
The last of Dimitrov's eight titles came when he won the ATP Finals on his debut in 2017, but it was not to be for the Bulgarian world number 33 who never recovered his rhythm after losing the tie-break.
"I'm having a great year, I'm very happy," Jarry said afterwards.
It was a third straight scalp for Jarry. He knocked out top seed and world number four Casper Ruud, the 2021 and 2022 defending champion, in the quarter finals. The Chilean then beat Alexander Zverev in the semis.
The 28-man Geneva tournament serves as a timely tune-up for the second Grand Slam of the year in Paris.
The Geneva title comes with 85,605 euros in prize money, while the runner-up earns 49,940 euros.
