St Johns Ambulance crews attended Port Beach in Fremantle at 10am local time to tend to the victim.
Police boats and lifesavers are searching for the victim in the water.
Their condition is not known.
Police boats and other vessels are in the water.
Port Beach was quickly closed, followed by Leighton Beach and Cottesloe Beach.
Surf Life Saving reported an unknown species of shark about two metres long had been spotted off Port Beach just after 10am.