Mandurah Detectives are investigating an alleged one-punch assault at the Monkey Bar and Lounge overnight.
It's understood around 10pm, a physical altercation occurred between a group of people outside the licensed venue on Pinjarra Road.
Police said during this time the 36-year-old man was punched by another man which caused him to fall to the ground.
The victim allegedly received serious injuries as a result of the incident and was treated by St John Ambulance at the scene.
He was then conveyed to the Intensive Care Unit at Royal Perth Hospital.
The other man fled the scene before police arrived.
Detectives are appealing for anyone who witnessed the incident, or with information regarding this incident, to contact Crime Stoppers by calling 1800 333 000 or by making a report online at www.crimestopperswa.com.au