It is the latest in a spate of shark attacks in the country and comes just days after a surfer was bitten in an attack in the same state.
A man was killed by a shark as he swam off a Western Australian city beach Friday while beachgoers watched in horror.
Boats and helicopters searched the ocean off Sorrento Beach in Perth's northern suburbs after the late-morning attack, a police statement said. Police would not say whether potential remains had been found.
But police were confident the "swimmer died as a result of the attack," the statement said.
A witness to the attack, identified only as Darrel, told Nine Network television that the victim had been swimming with a now traumatized friend.
"I was just shocked. I mean, I feel for his mate. He's apologized and said 'I didn't save my mate,'" Darrel told Nine.
"I said 'You couldn't have done anything,'" Darrel added.
Witness Ingrid Mitchell, who swims at the beach daily, said to Nine: "How do you just watch someone be mauled to pieces in front of you, basically? It's pretty hard."
Witnesses reported seeing blood in the water.
"My friend said, 'Jac, I think there's been a shark attack,' and I looked out to the ocean and I could see a pool of blood," beach walker Jacqui Rapaic told Australian Broadcasting Corp.
Local Ryan Rowe told The West Australian newspaper that he reached the beach soon afterward and was told by an onlooker "someone had been bitten by a shark."
"I couldn't believe there was that much blood (that had) come out of a human body," Rowe said.
Authorities closed the beach to swimmers. Witnesses described the swimmer as a man in his 60s but police did not identify him further.
In a separate shark attack, a surfer sustained a serious leg injury Monday near Geraldton, about 400 kilometers (250 miles) north of Perth.
There have been at least five fatal shark attacks in Australia this year.
A 12-year-old boy died after he was bitten by a shark while swimming in Sydney Harbour in January. The attack that killed the boy was one of four recorded in the span of two days, which prompted officials to close dozens of the city's beaches.
Three divers were also fatally mauled in separate incidents between May and June — two in Western Australia and the third in Queensland.
Also in June, a woman was mauled by a shark at one of Australia's most popular beaches in Sydney. The woman — a schoolteacher — last month recalled swinging a "left hook" at the predator before it dragged her underwater.
Australia has averaged between two and three fatal shark attacks a year since 2000, according to the Australian Shark Incident Database, a partnership including Taronga Conservation Society Australia, Flinders University and the New South Wales state government.
The International Shark Attack File, a database of global shark attacks maintained by the University of Florida, has recorded more than 1,280 shark incidents around Australia since 1791, including more than 250 fatal incidents. Such incidents have become more common as the population has grown and activities such as surfing and scuba diving have gained popularity.
Agence France-Presse contributed to this report.