The work of art was found inside a suitcase in the bus’s luggage compartment by French customs agents during a stop-and-search in Marne-la-Vallee, where Disneyland Paris is located.
Agents were surprised to find the 1877 piece with Degas’ signature, the French Culture Ministry said on Friday.
None of the passengers claimed the suitcase during the search on 16 February, it added.
Experts from the Musee d’Orsay verified the piece as Degas’ Les Choristes (The Chorus Singers), which depicts a scene from Mozart’s opera Don Giovanni.
The painting was stolen from a museum in Marseille in 2009 while on loan from the Musee d’Orsay in Paris.
French culture minister Francoise Nyssen said she was delighted with “this happy rediscovery of a precious work belonging to the national collections, whose disappearance represented a heavy loss for the French Impressionist heritage”.
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Degas is regarded as one of the founders of Impressionism, although he preferred to be called a realist.
More than half the Parisian’s works depict dancers.