The National Highway Transportation Safety Agency (NHTSA) yesterday said the ruling -- which won't apply to urban transit or school buses -- would go into effect in November 2016.
"Adding seat belts to motorcoaches increases safety for all passengers and drivers, especially in the event of a rollover crash," said NHTSA chief David Strickland.
On average, 21 occupants of inter-city coaches and other large buses are killed, and nearly 8,000 injured, in motor vehicle accidents every year, the NHTSA said.
The worst bus accident in US history occurred in May 1976 when a school bus carrying a high school choir went off a cliff in Yuba City, California.
Seat belts have been mandatory in US cars since 1968, but the NHTSA says they're not needed in school buses, which are subject to their own stringent safety and design guidelines.