The 53-year-old victim was driving the Route 16 bus at about 1:15 a.m. Saturday when a group of 10 to 15 youths boarded the bus and started punching him, The Boston Herald reported.
The bus driver was treated for injuries at the scene of the incident, the newspaper said.
Police said there is no video footage of the incident, but they have talked to several witnesses. They have not yet determined a motive for the attack.
"We have seen an uptick in assaults on bus operators, but typically they involve disputes over fares," said MBTA police Superintendent in Chief Joseph O'Connor. "In this case, it's highly unusual."