The colourful image of singers performing on a theatre stage was missing when staff opened up the Cantini Museum for the day, prosecutor Jacques Dallest said.
The national museums service said the picture - a pastel work titled The Chorus, was worth 800,000 euros (US$1.14 million), correcting an estimate given by police and museum staff that it was worth some 30 million euros.
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Loaned by the Orsay museum in Paris for an exhibition of theatre-themed artworks including some 20 works by Degas, it measures 32 centimetres by 27. Dallest said it had been unscrewed from the wall.
'Inside job' suspected
"As far as I know there was no break-in," Dallest said, adding that investigators suspected an intruder, a visitor to the exhibition or an inside job.
City councillor Maurice Di Nocera, responsible for organising major events in Marseille, called the theft "a disaster for the museum".
The museum was closed on Thursday while police investigated, examining film from security cameras.
Marseilles city hall, which runs the museum, said 70,000 people had visited the exhibition and the show was due to close on January 3 before touring to Italy and Canada.
