At 21 years and 10 months, Merida becomes the third-youngest winner of the Croatian tournament, joining illustrious company in compatriot Carlos Alcaraz (18 years, two months) and Italian world number one Jannik Sinner (20 years, 10 months).
"Two of the best guys in the world, so it's amazing to be in this group of players," Merida said.
"Of course they are far away, but I will keep working to try and get as close as possible to them.
"It was a crazy match... But I'm super happy with my level this week and for my first title."
Having been the runner-up at the ATP 250 tournament on clay in Bucharest in April, Merida's first title will also earn him a new career-high ranking of 58.