The top seed secured his first clay-court title with victory over the Uruguayan in one hour and 43 minutes.
"You really have to fight hard and give your soul out on the court," said Tsitsipas. "This title means a lot to me. It's on clay, it's one of my preferred surfaces.
"It's nice to have completed the clay-hard court title [sweep] that I've been fighting for. Next is grass, or maybe even more clay-court titles this year, that would be wonderful."
Tsitsipas will head into back-to-back ATP Masters events after winning titles in Stockholm, Marseille and now Portugal.
"I've been building my game. It hasn't been an easy transition from hard to clay this year, so I've been trying to play as many matches as I can before the big events start," he said. "Madrid obviously is going to be a huge challenge for me. I really want to do well, get as many points as possible, Rome as well.