AMSTERDAM — For roughly a century, the painting “Sunset at Montmajour” was considered a fake. It was stored in an attic and then held in a private collection, unknown to the public and dismissed by art historians. But on Monday, the Van Gogh Museum declared the work a genuine product of the master, calling it a major discovery.

“Sunset at Montmajour,” painted in Arles in 1888, “is a work from the most important period of his life, when he created his substantial masterpieces, like ‘Sunflowers,’ ‘The Yellow House’ and ‘The Bedroom,’ ” said the museum’s director, Axel Rüger, in an interview. The painting depicts dusk in the hilly, forested landscape of Montmajour, in Provence, with wheat fields and the ruins of a Benedictine abbey in the distance. The area around Montmajour was a subject that van Gogh revisited often during his time in Arles.

“One or two early van Goghs do sometimes come out of the woodwork now and again, but from the mature period, it’s very rare,” said James Roundell, an art dealer and the director of modern pictures for the Dickinson galleries in London and New York, which deals in Impressionist and modern art.

Mr. Roundell said it would be hard to predict precisely how much “Sunset at Montmajour” would fetch on the market, but expected it would be “in the tens of millions and quite a few of them.”