A relative found the 77-year-old man with serious injuries on Monday at his home in Redmond, about 400km (250 miles) south of Perth.
When paramedics arrived at the rural property, the kangaroo prevented them from treating the man.
Police were forced to shoot the marsupial dead. The man died at the scene.
A police spokesperson told media they believed the man had been attacked by the kangaroo – a wild animal – earlier in the day.
Australia is home to about 50 million kangaroos, which can weigh up to 90kg and grow to 2m tall.
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But fatal attacks are rare – this is the first one reported in Australia since 1936.
Kangaroos have “a lot of weapons” such as sharp teeth, claws and powerful legs, kangaroo behaviour expert Graeme Coulson told the Australian Broadcasting Corporation.
“Certainly if they’re cornered or in some sort of distress, that can be quite dangerous,” Associate Professor Coulson said.
“The problem with kangaroos and people is we’re both upright animals, we stand on our two feet, and an upright stance like that is a challenge to the male kangaroo.”
In July, a kangaroo left a 67-year-old woman with cuts and a broken leg after it attacked her on a walk in Queensland.
And a three-year-old girl suffered serious head injuries in an attack in New South Wales in March.
Urban development across Australia is increasingly encroaching on wild kangaroo habitats.
