It happened on Saturday at about 11:22 p.m. in the 5700-Block of S. Ashland Avenue, according to police.
Chicago police said a group of three men attempted to rob a man on the bus.
The victim tried to defend himself and pulled out a sharp object, police said.
A witness, who did not want to be on camera, said after they robbed him, they threatened to "blow his brains out."
Police said the victim had a cut to his hand.
He was taken to hospital to be treated, and at last check was expected to be OK.
Chicago police said the three suspects were taken into custody shortly after the attack.
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