Police say they were called to a housing complex in that Vancouver Island city at about 1:45 p.m. Sunday.
Officers found a man with life-threatening stab wounds and the victim was rushed to hospital but could not be saved.
Investigators say a suspect was arrested a short time later and is expected to appear in court in Nanaimo today.
Mounties are not speculating on a motive for the stabbing.
They have not said if the victim and suspect were acquainted.