The fatal attack happened at Rottnest Island, near Perth, about 9.55am on Saturday.
"A 38-year-old man was in the water at Horseshoe Reef when he was believed to have been bitten by a shark," Western Australian Police said.
The man was taken from the water and transported by boat to a jetty at Geordie Bay on the north side of the island.
Paramedics treated the critically injured man.
"Sadly, the man was unable to be revived," the police spokeswoman said.
A five-metre white shark was spotted 80 metres offshore about the time of the attack, according to Surf Life Saving WA.
Television news footage showed paramedics and police treating the wounded man on the jetty.
It also showed a Water Police boat tethered to a smaller boat equipped with spearfishing gear.
The Department of Primary Industries and Regional Development urged people to take extra caution in the Geordie Bay area.
Rottnest Island, which is 11km long and 4.5km wide, is a popular tourist destination 19km off the coast of Fremantle.
Police will prepare a report for the coroner.
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