A Haystack in the Evening Sun had not previously been authenticated because the work is largely unknown and the artist's signature is covered by paint.
However researchers at the University of Jyvaskyla in Finland uncovered the signature using a hyperspectral camera.
It also revealed the date of the work's creation - 1891.
The special camera used by researchers studied the painting's elemental composition by measuring X-ray fluorescence.
That allowed them to "see" below the surface, and analyse the materials used to create the work.
"The camera is principally operating as a scanner, which scans one line at a time," researcher Ilkka Polonen said.
"When the camera is moved using the scanner, an image of the whole picture can be obtained."
An analysis of the pigments and canvas fibres also confirmed the painting was by the Impressionist.
The artwork is currently owned by Finland's Serlachius Fine Arts Foundation, which acquired it in the 1950s through a London art broker.
The institution said the authentication means the artwork is the first Monet painting to be held in a Finnish public collection.
An oil painting thought to have been created by French Impressionist Claude Monet has been proven to be genuine through scientific testing.