Officers and prison medical staff provided first aid to the 32-year-old after he was discovered by guards on Sunday afternoon, Western Australia's Department of Justice says.
Paramedics arrived by ambulance but the man was declared deceased at the prison. There were no suspicious circumstances, the department said in a statement on Monday.
West Australian police will provide a report to the coroner.
A 29-year-old prisoner died in hospital in May after he was found unconscious in a kitchen change room at Albany Regional Prison in WA's south.
Another Indigenous man, aged 22, died at Perth's Hakea remand prison in March.
There were no suspicious circumstances in either of those deaths, according to the department.
More than 500 Indigenous people have died in custody since the 1991 Royal Commission into Aboriginal Deaths in Custody.
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