Police say the suspect starting yelling at a man and woman, on Centre Street around 7:00 a.m. and that verbal confrontation escalated into a fight.
They say the male victim was injured and taken to hospital, while the suspect ran away westbound, through the parking lot beside the post office.
The victim has since been released from hospital.
Investigators are describing the suspect as a white man, about six feet tall, with a heavy build and brush cut.
He was wearing grey track pants.
Anyone with information can call Napanee OPP or Crimestoppers.