Ulrich Leisinger, head of research at the International Mozarteum Foundation in Salzburg, Austria, said Thursday that there is no doubt that the single sheet was written by the composer and that it is "really important."
He described the work as the preliminary draft of a musical composition. He said it was found by a library in Nantes, western France, as staff were going through its archives.
The city is planning to hold a news conference on the find later on Thursday.
Leisinger said the library contacted his foundation for help authenticating the work.
The sheet was bequeathed to the library by an autograph collector in the 19th century and was catalogued back then as part of the library's collection, he said.
But it was later "entirely forgotten," essentially becoming lost to scholars for more than a century, and was only rediscovered by the library as it re-catalogued its archives in recent years. It was unclear what happened to the library's 19th century catalogue.
"This is absolutely new," Leisinger said in a telephone interview. "We have new music here."
"His handwriting is absolutely clearly identifiable," he added. "There's no doubt that this is an original piece handwritten by Mozart."
There have been about 10 Mozart finds of such importance over the past 50 years, he said. If sold, the single sheet would likely be worth around US$100,000 (euro70,000).
"The fact that an entirely new sheet shows up is extremely rare," he said.
Circumstantial evidence, including the type of paper, suggests Mozart did not write it before 1787, Leisinger said. Mozart died in 1791.
Mozart was interested in church music at that time and was planning to become the choir and music director of Vienna's main cathedral, although he died before he could take up the post.
In all, about 100 such examples of musical drafts by Mozart are known. Many are notes for works that he went on to complete.
But the rediscovered sheet is the "draft for a piece that Mozart did not work out for whatever reason," said Leisinger.
"It's a melody sketch so what's missing is the harmony and the instrumentation but you can make sense out of it," he said. "The tune is complete. It's only one part and not the whole score with eight or twelve parts."
"One can really get a feeling of what Mozart meant although we do not know how he would have orchestrated it."
The sheet appears also to have been examined in the 19th century by Aloys Fuchs, a well-respected autograph hunter who collected works from more than 1,500 musicians.
Fuchs wrote, "authenticity of this present handwriting of W.A. Mozart is confirmed," in an annotation dated Aug. 18, 1839, in Vienna.