Prosecutors believe Trevonnte Brown was wearing a ski mask when he boarded the bus last Monday night near the intersection of 3rd and Pike in downtown Seattle.
As the bus neared West Seattle, Brown allegedly held up three passengers at gunpoint, robbing them of their cell phones, before the third victim - and several other passengers - tackled him and held him down, authorities said.
Brown, a recent graduate of Franklin High School with no criminal history, was arrested and is being held on $350,000 bail.
He could face more than 15 years in prison if convicted. His arraignment is scheduled for December 16.