An inmate suffered burns and was taken to hospital after a fire broke out at Hakea Prison this afternoon.
A police spokesman said up to four other people, including staff members and inmates, were treated for smoke inhalation after the fire started in a cell.
He said some of those injured were treated by St John Ambulance paramedics, while others were treated by prison medical staff.
A Fire and Emergency Services Authority spokesman said 20 firefighters attended the blaze, which broke out just before 4.40pm and was extinguished about 30 minutes later.
The fire was contained to the cell.
A St John Ambulance spokesman said two prison guards and three inmates were taken to Royal Perth Hospital.
Police are at the scene investigating the cause of the fire.