Which Portuguese national park is the only one?

Peneda-Gerês National Park is Portugal’s only designated national park. It was created on May 8, 1971, in the mountainous northwest of the country near the Spanish border.

The park’s name comes from two of its major mountain ranges, the Serra da Peneda and the Serra do Gerês. It also includes the Serra Amarela and Serra do Soajo, along with oak and mixed forests, rocky uplands, waterfalls, rivers, and traditional rural villages. The protected area covers roughly 700 square kilometres.

Peneda-Gerês is notable for both natural and cultural history. Roman engineers built the Geira road through the region to connect Bracara Augusta, modern Braga, with Asturica Augusta in Spain. The park also contains the remains of Vilarinho das Furnas, a village flooded in 1970 after a dam was built on the Homem River; its ruins can reappear when water levels fall.

Portugal has many nature parks, including Arrábida and Serra da Estrela, but those have a different legal designation. That distinction is why Peneda-Gerês is the answer to this question.