It was a career-first ATP title in front of home support for Davidovich Fokina, the world number 25.
The result means the 27-year-old Davidovich Fokina is now 1-5 in tour-level finals.
After five final defeats -- including four in 2025 -- Davidovich Fokina fought through in one hour and 43 minutes in his first meeting with the 63rd-ranked American.
"It feels amazing to have the first title," said Davidovich Fokina who sealed victory with his sixth ace of the match.
"Today was a very tough battle, he played amazing. In the end I knew that this one has to be mine, in Spain, at home, the first one. It should be mine and I pushed a lot.”
With this maiden ATP title, the Spaniard will rise to world number 23 on Monday as Wimbledon begins, while 22-year-old Quinn will reach a career-high ranking of 47.
In the first round of Wimbledon, Davidovich Fokina will face Argentine Juan Manuel Cerundolo, while Quinn will play against another Argentine Marco Trungiletti.