The 22-year-old prisoner allegedly murdered at HMP Lindholme over the weekend suffered a fatal stab wound.
A post-mortem examination carried out last night revealed the cause of death and it is hoped a formal identification will be carried out later today.
South Yorkshire Police was contacted by officials from the prison at around 3.50pm on Saturday, December 21, and informed that an inmate had died.
Detectives are investigating the death and have arrested two men, aged 23 and 26, on suspicion of murder. They have since been released from police custody on bail and returned to the prison service pending further enquiries.