A 25-year-old man was found dead in his cell at Corradino Correctional Facilities on Thursday morning.
The Agency for Correctional Services said the man, a Pakistani national, was found by correctional officers at 6am.
Nothing of a suspicious nature was found in his cell so initial indications are that he died while sleeping, the agency said.
It said the police were immediately informed and an inquiry is being held.
More than 10 prisoners have been found dead in their cell in two years.
PN statement
In a statement, the Nationalist Party said this latest death was another reminder that the Robert Abela led government had failed in absolute terms in the administration and reform of the prisons.
All who believed in human dignity were concerned and worried at how, faced with yet another death in prison, Home Affairs Minister Byron Camilleri still did not understand that there was a crisis in the facility's administration and reform and failed to take action for such tragedies to stop.
All lives, even those of prisoners, were precious and no one should lose theirs because of systems, methods and failed ways of administration, the PN said.
It said the minister’s lack of action rendered him politically responsible for the tragedy the prison was going through.