"Sunset at Montmajour" depicts trees, bushes and sky, painted with Van Gogh's familiar thick brush strokes. It can be dated to the exact day it was painted because Vincent described it in a letter to his brother, Theo, and said he painted it the previous day -- July 4, 1888.
He said the painting was done "on a stony heath where small twisted oaks grow."
Museum experts said the painting was authenticated by letters, style and the physical materials used, and they had traced its history.
Museum director Axel Rueger described the discovery as a "once-in-a-lifetime experience" at an unveiling ceremony.
The museum said the painting now belongs to an unidentified private collector and will be on display at the museum from Sept. 24.
It did not disclose full details of how the painting had been recovered, but said that it had been owned by a Norwegian man who had been told it was not by Van Gogh, so he put it in the attic.
Rueger said the museum had itself rejected the painting's authenticity in the 1990s, in part because it was not signed. But new research techniques and a two-year investigation had convinced them.
Researcher Teio Meedendorp said he and other researchers "have found answers to all the key questions, which is remarkable for a painting that has been lost for more than 100 years."
The painting was listed among Theo van Gogh's collection as number 180, and that number can still be seen on the back of the canvas. The work was sold in 1901.
Vincent Van Gogh struggled with bouts of mental distress throughout his life, and died of a self-inflicted gun wound in 1890. He sold only one painting while he was alive, though his work was just beginning to win acclaim. The Van Gogh Museum, which houses 140 of the Dutch master's works, receives more than a million visitors annually, and Van Gogh paintings are among the most valuable in the world.
Rueger described "Sunset" as ambitious, because the canvas is relatively large, at 93.3 by 73.3 centimeters (36.7 by 28.9 inches).
Van Gogh referred to the work in two other letters in the same summer it was painted, but he said he considered it a failure in several respects.
The location it depicts can be identified: it is near Arles, France, where Van Gogh was living at the time, near Montmajour hill, and the ruins of an abbey of the same name. The ruins can be seen in the background of the work, on the left side.
Researcher Meedendorp said it belongs "to a special group of experimental works that Van Gogh at times esteemed of lesser value than we tend to nowadays."
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