The two paintings - Le Jeu de la Palette and Le Jeu de la Bascule - have been declared national treasures and banned from export for the next 30 months, to give French collectors or museums the chance to buy them.
Their value is estimated at six million euros.
They were discovered in a château in Normandy by Thadée Prate, an art specialist at the Tajan auction house, when he and colleague Nicolas de Moustier were invited to value its contents, Le Figaro newspaper revealed on Saturday.
Spotting the paintings hanging among many others 1.90 metres above ground, he climbed a ladder to take check his hunch they were by the 18th-century French master.
Le Jeu de la Palette and Le Jeu de la Bascule disappeared in 1786, when they auctioned following the death of one of Fragonard's patrons, Bergeret de Grandcourt, who had commissioned them.
Pointing to Italian influence in their style, Prate believes the artist painted them after his return from a visit to Rome in 1761 at the age of 29.
In December Prate identified Leonardo as the author of a drawing of Saint Sebastian, a judgement confirmed by an independent expert.
Both the paintings and the drawing, which has also been temporarily banned from export, will be on show at the auction house on 9-10 June.