Emergency services attended Perth's Sorrento Beach about 10.30am on Friday after reports a male had been bitten by a shark while swimming along the coast.
Authorities searched for the swimmer from the air and sea, but he died from the attack, with his remains found in the water, WA Police said.
The Department of Primary Industries and Regional Development is investigating the incident, the second shark attack off WA's coast in five days.
A witness told the ABC she was alerted to the attack when she was walking along the beach and saw a pool of blood in the water.
The City of Joondalup has closed all beaches in the area, from Burns Beach in the north to Marmion in the south, until midday on Saturday as a precaution.
The fatality follows a shark attack on a surfer at a remote WA beach on Monday.
The man was seriously injured at Glenfield Beach, north of Geraldton in the state's Mid West
He was taken to the nearby Geraldton Health Campus and is now in a stable condition.
Surf Lifesaving WA's helicopter shark patrols began for summer last week.
They will operate initially on weekends before increasing to a daily frequency between Mandurah and Yanchep, and Rottnest Island to the west, from September 26 until the end of April.
Patrol flights in WA's southwest between Bunbury and Hamelin Bay will run daily during school holidays and peak summer periods, and on weekends and public holidays at other times.
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