Subhan Anwar, who was jailed for a minimum of 23 years in 2009 for the murder of Sanam Navsarka, is understood to have been held hostage in a cell at HMP Long Lartin in Worcestershire before he was killed.
His partner Zahbeena Navsarka was cleared of her daughter’s murder but found guilty of manslaughter and jailed for nine years.
A Prison Service spokeswoman said: “An incident at HMP Long Lartin on Thursday February 14 resulted in the death of prisoner Subhan Anwar.
“Two prisoners are now in police custody and the matter is being investigated by police.”
Anwar and Navsarka, from Huddersfield, were condemned for their cruel and selfish treatment of Sanam.
The violence inflicted on the youngster happened near the end of her life.
Her tiny hand prints and bloodstains were found inside cupboards at the home in Huddersfield and also at a former property in Batley.
Both her thigh bones had been fractured, causing fatty deposits to enter her bloodstream and resulting in her death, and she had fractures to both her arms.
A West Mercia Police spokesman said: “West Mercia Police has launched a murder investigation after a 24-year-old man died in Long Lartin prison last night.
“The body of the man was found by prison officers just before 8pm on Thursday, February 14.
“Two men aged 47 and 45 have been arrested on suspicion of murder and are currently in police custody.
“West Mercia Police is working closely with the Ministry of Justice to establish the exact circumstances surrounding this man’s death but it is unlikely any further comment will be made today.”