A 38-year-old Palestinian security prisoner died in Ketziot Prison, the Prison Service announces.
He is identified as Tair Abu Asab, a member of the Fatah movement, who had been jailed since 2005 on charges of attempted murder relating to a terror attack.
The circumstances of his death were being investigated, the service says.
In a separate announcement, the service says that a short while later riots broke out in Ohalei Keidar Prison in Beersheba, leading to an emergency being declared in the jail.
Prisoners set their mattresses and clothes on fire.
Wardens managed to put out the fires and take control of the wards, the service says, noting that one guard was lightly injured by smoke inhalation.