It was not immediately clear what caused the violence, which has largely been restricted to the province's capital, Assosa, although residents told Reuters that large mobs were attacking ethnic minorities that had settled there.
The Berta and Gumuz ethnic groups are considered indigenous to Benishangul-Gumuz, which borders Sudan, but the province has a large presence of ethnic Amharas and Oromos, who are labelled "highlanders" and originally hailed from the country's larger Amhara and Oromiya provinces, Presstv Reported.
Ashadli Hassen, the region's president, said nine people have died in the violence so far. A day earlier, officials had said only one person was killed.
"We have 40 suspects in custody and we believe they have been given financial incentives to commit violence," he told reporters on Thursday. He did not give further details.
The violence comes amid increasing ethnicity-based unrest throughout the country and after a grenade attack at a rally on Saturday that Prime Minister Abiy Ahmed attended.