Darren Salituri has died at Les Nicolles Prison.
The 39-year-old was confirmed to have died in a statement released to the media on Monday lunchtime.
John De Carteret, the Prison Governor, said Mr Salituri had died earlier that day.
An investigation is now underway to determine what happened, he said.
“This morning (Monday 2 February) a prisoner – Darren Salituri, aged 39 – was found deceased in his cell by prison officers,” explained Mr De Carteret.
“The prison has notified Guernsey Police, who are now investigating the circumstances of the death, and the Guernsey Prison Service is also now undertaking its own internal investigation.
“Firstly it is important to state that any death in prison is a tragedy, and we will now work closely with the police to establish the full circumstances of the incident. This death will undoubtedly have an impact on many, not least his family. My thoughts are with them at this time.”
On behalf of Guernsey Police, Detective Chief Inspector Ian Tanner said officers are now working with both Mr Salituri’s family and his victims.
“Guernsey Police attended Guernsey Prison this morning, responding to reports of a deceased prisoner,” he confirmed.
“The death is not currently being treated as suspicious, and we are undertaking a coronial investigation in line with standard practice.
“Officers have this morning made the family of the deceased, and the victims of his offending, aware.
“Enquiries are ongoing.”
Mr Salituri was well known in Guernsey. He was recognisable by his facial tattoos, and he was often accompanied by his dog.
He had worked to clean up gravestones in recent years, offering his time to other people for free on some occasions.
It is not currently clear what Mr Salituri was in prison for at the time of his death.
In the past six years he has served prison sentences for harassment and dangerous driving, and possession of a knife.