The bear, which most likely belongs to a population living in the western Cantabrian Mountains in northern Spain, is thought to have wandered across the border.
“The reappearance of individuals from this species in Portugal… has now been confirmed by the ICNF,” the Portuguese Institute for Conservation of Nature and Forests (ICNF) said.
Brown bears have been extinct in Portugal since the 19th century.
“The last reports of a stable presence of brown bears in Portugal are between the 18th and the end of the 19th century. They then died out,” the ICNF said.
The animal was spotted in the Montesinho Natural Park and Braganca commune in northeastern Portugal.
?A última vez que se vira un oso en #Portugal foi no Gerês, en 1843. O Instituto de Conservação da Natureza e das Florestas (ICNF) e a Junta de Castilla y León confirman a presenza dun exemplar na #Teixeira, no municipio de #Hermisende. Máis info ⬇️https://t.co/2weTkUvNYt pic.twitter.com/Ie2hTrktZZ
— GCiencia (@GC_GCiencia) May 9, 2019
The town of Bragance is about 20 kilometres (12 miles) from the Spanish border.
Media reports say the last living bear in Portugal was killed in 1843 in the northwest mountainous region of Geres.
“The fact that a bear has crossed our border does not mean that there is a bear established in Portugal. At the moment we have a stray animal,” said Paulo Caetano, an author of a book on bears, on Portuguese radio.
The animal is probably a young male looking for “a peaceful territory, a companion and food,” he said.
The bear population in the Cantabrian mountain range, which extends east to west over four Spanish regions, has been increasing since the 1989 adoption of a relocation plan.
In 2018, some 330 bears were counted in the mountains there, according to the environmentalist foundation Oso Pardo.