The painting is believed to be Renoir's "Paysage Bords de Seine," a 5.5-by-9-inch pastel river scene. The work has been valued at between $75,000 and $100,000, The New York Times reported.
The owner, who wishes to remain anonymous, brought the painting to the Potomack Company, an auction house based in Alexandria, Va., where Anne Norton Craner, the company's fine arts specialist, said she is convinced it is a Renoir.
"You just see it and you know it's right," Craner said.
Craner said she did some research and found the painting was purchased from the Bernheim-Jeune gallery in France in 1925 and later sold to Herbert May, the husband of Sadie A. May, a collector in Maryland.
How the painting made its way from the Mays' collection to a box at a flea market is a mystery, Craner said.
The painting is set to be auctioned off on Sept. 29.