Mariusz Krezolek and Daniel's mother Magdelena Luczak were convicted of the boy's murder in 2013, and jailed for 30 years.
A judge said the four-year-old was the victim of an "incomprehensible cycle of cruelty and violence" which left him looking like "a concentration camp victim" before he died of a head injury in 2012.
Krezolek was discovered unresponsive in his cell at HMP Full Sutton in Yorkshire on Wednesday morning.
A Prisons Service spokeswoman said: "Staff immediately attempted resuscitation but he was pronounced dead shortly after."
An investigation will be carried out by the independent Prisons and Probation Ombudsman.
Luczak, 29, was found dead at HMP Foston Hall in Derbyshire in July last year - two years into her sentence.
The cause of death was recorded as hanging.
During his final month, Daniel was denied food, forced to perform punishment exercises, confined in a locked box room, poisoned with salt, and subjected to water torture.
He weighed just 1st 9lb when he died.
The medical evidence, the judge said, showed that Daniel's emaciation was regarded by experts as "unprecedented" in Britain.
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