Until his transfer, Duch had been staying at the ECCC Detention Unit.
Staff members had said the accommodation at Kandal provincial prison "is that which will best protect his (Duch's) interests," the prosecutors' office said in a statement.
The tribunal last year sentenced Duch to life imprisonment for war crimes and crimes against humanity.
It said Duch oversaw the killing of at least 12,272 victims but some estimates have placed the number as high as 16,000.
The ultra-Maoist Khmer Rouge movement ruled Cambodia between 1975-79 when an estimated 2 million people died from execution, overwork, starvation and disease.