A 19-year-old inmate has been charged over the death of 35-year-old Troy Roginson at Wooroloo Prison Farm on Monday.
It is understood Mr Roginson, a convicted burglar, was found unconscious by another inmate at the minimum-security prison just before 6pm.
He was taken to hospital with serious head injuries but died on Tuesday night.
Police said the incident was being treated as a death in custody.
The 19-year-old inmate has been charged with one count of unlawful assault causing death.
He is due to appear in the Perth Magistrate's Court today.