An officer found Bell, 40, lying in the street, Atlanta police said. Another man nearby had also been shot but survived.
The survivor, who was taken to a local hospital, told police he and Bell were shot after first being robbed when they stepped off the bus, police said.
No arrests have been made, police said.
Born in Jamaica and known as "Supernova" throughout his career, Bell was the undisputed cruiserweight champion in 2006, winning the World Boxing Council, World Boxing Association and the International Boxing Federation titles after a 10th-round knockout of Jean-Marc Mormeck at Madison Square Garden in New York.
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Bell lost a unanimous decision to Mormeck the following year in France.
Bell had a 27-4-1 career record, including 25 victories by knockout. His last appearance came in December 2011, when he beat Rico Cason in a first-round knockout.
(Additional reporting by Steve Ginsburg; Editing by Karen Brooks and Steve Orlofsky)
