The 22-year-old victim was sitting on the back of a truck near his home on the 900 block of West 42nd Street, police said.
A man came up behind the victim and demanded he give up everything he had, investigators said.
Police said the victim tried to hit the other man, but missed.
The robber cut the man on the arm and left, according to police.
The victim was taken to Regional Medical Center for treatment. Police did not consider his injury life-threatening.