The victim is believed to have been bitten by a white shark near Cull Island, which lies close to the mainland’s Esperance coast.
The attack happened as the man was diving from a boat just after 1pm on Sunday.
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Marine police are still searching for his body. It is not yet known if the man was a local or a tourist.
A woman who was on board the boat has been treated in hospital for shock.
The Department of Primary Industries and Regional Development confirmed the attack on Sunday afternoon, saying “a man received fatal injuries after being bitten by a reported white shark.”
Members of the public are being urged to take additional caution while in the area and report any shark sightings to authorities.
Fisheries officers will also be conducting patrols in the area.
A police spokesman said: “Esperance Police and Marine Rescue WA volunteers deployed to the area on board the marine rescue vessels. They are still searching the ocean for the victim.”
Australia is known for a large number of shark encounters with humans.
The attack on Sunday was the second fatality in the area in less than three years, although there have been fatal attacks elsewhere in the country since then.
In April 2017, 17-year-old Laeticia Brouwer died after being attacked by a great white shark while surfing with her father off the Esperance coast.
In October, a British man had his foot torn off and another has had his leg badly injured in a shark attack while the pair were snorkelling on Australia’s Great Barrier Reef.
Additional reporting by Reuters and AP