Unidentified gunmen opened fire and killed the driver, before setting the bus on fire, according to authorities.
Most of the victims were women who were on their way to work at the market, local sources told the EFE news agency.
Armed men "shot at the vehicle, slashed the tires, and shot at the people", Moulaye Guindo, the mayor of the nearby town of Bankass, told Reuters news agency.
He said more people were either missing or wounded.
The bus was targeted as it made its twice-weekly journey from the village of Songo to a market in Bandiagara, 10km (6.2 miles) away.
The attack took place at the heart of the West African nation's Mopti region, which has seen escalating violence in recent months fuelled by fighters linked to Al-Qaeda and ISIL.
ZZ/PR
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