The attack happened at 8.45am at Gnarabup Beach, the state’s Department of Primary Industries and Regional Development (DPIRD) said.
The attack happened at the Boat Ramp surfing spot.
DPIRD said it was suspected a great white shark was behind the bite.
Local beaches from Gas Bay to Margaret River Mouth have been closed.
According to the Sharksmart WA website, a great white was spotted in the area just after 9am.
However, last week water users were warned the Sharksafe WA Twitter alerts were not working due to a “technical fault” and that people should check the app or website.
St John’s Ambulance WA said it wasn’t called.
Swimmers have been told to be careful at the beach, which is three hours south of Perth.
Sharks spotted metres from the shore at beach in Queensland