PARIS—It’s a forgotten melody, sketched in black ink in a swift but sure hand.

The single manuscript page, long hidden in a provincial French library, has been verified as the work of Mozart, the apparent underpinnings for a Mass he never composed.

The previously undocumented music fragment gives insight into Mozart’s evolving composition style and provides a clue about the role religion may have played for the composer as his life neared its turbulent end, one prominent Mozart expert says.

A library in Nantes, western France, has had the fragment in its collection since the 19th century, but it had never been authenticated until now, partly because it does not bear Mozart’s signature.

Ulrich Leisinger, head of research at the International Mozarteum Foundation in Salzburg, Austria, said Thursday that there is no doubt that the single sheet, the top third of which has been cut off, was written by the composer.

“His handwriting is absolutely clearly identifiable,” he added. “There’s no doubt that this is an original piece handwritten by Mozart.”

Leisinger said the work had been “entirely forgotten.” Such a find is rare: The last time unknown music in Mozart’s handwriting came to light was in 1996, when a portion of an aria was sold at Christie’s, Leisinger said.

The library does not plan to sell, but if it did, the single sheet would likely be worth around $100,000, the expert said. In all, only about 100 such examples of musical drafts by Mozart are known.

There have been up to 10 Mozart discoveries of such importance over the past 50 years, Leisinger said.

The sheet was bequeathed to Nantes’ library by a collector in the 19th century, along with one letter from Mozart as well as one from his father. Both the letters were published in Mozart’s complete correspondence, said Agnes Marcetteau, director of Nantes’ municipal library.