Degas artwork stolen from museum
Les Choristes (The Chorus) was missing when staff at the Catini Museum opened the premises on Thursday morning.
There were no signs of a break-in, said Jacques Dallest, the French city's public prosecutor.
The museum will now remain closed while the theft is being investigated, including the examination of CCTV footage from security cameras.
Mr Dallest also told Agence France Presse that police are looking into the possibility that the artwork was stolen by an intruder, a visitor or a museum insider.
Les Choristes, which dates from around 1876, is a colourful pastel work which depicts a male line of singers in the midst of performance.