Magdalena Luczak and her partner Mariusz Krezolek were each jailed for a minimum of 30 years for murdering Daniel Pelka.
He was starved and beaten for months before he died from a head injury in March 2012 at his Coventry home.
Luczak, 29, died in her cell at HMP Foston Hall Prison in Derbyshire. The Ministry of Justice said the circumstances would be investigated.
She and her partner, both originally from Poland, had denied murder, blaming each other for Daniel's death.
The boy, who would have turned eight on Wednesday, weighed just over a stone and a half when he died.
He was denied food, forced to perform punishment exercises, confined in a locked box room, poisoned with salt, and subjected to water torture.
Birmingham Crown Court heard he saw a doctor in hospital for a broken arm, arrived at school with bruises and facial injuries, and was seen scavenging for food.
A teaching assistant said he was a "bag of bones".
Judge Mrs Justice Cox described his ordeal as "incomprehensible brutality".
The judge told Luczak and Krezolek they had caused Daniel "severe physical and mental suffering".
She said his punishments were "designed to humiliate" and that he was "a victim of chronic and systematic starvation" in a "campaign of cruelty".
In December three Court of Appeal judges threw out challenges by the pair. They rejected a bid by Krezolek to appeal against his murder conviction and dismissed claims from both that their sentences were "excessive".
Luczak was found unresponsive in her cell at about 07:15 BST.
A Ministry of Justice spokesman said: "Paramedics attended and she was pronounced dead at the scene.
"As with all deaths in custody there will be an investigation by the independent Prisons and Probation Ombudsman."
A serious case review found chances were missed to save Daniel, who was "invisible" at times.
The report concluded that "no professional tried sufficiently hard enough" to talk to him.
A GP, Dr Mohammad Pathan, was given a warning on 1 July this year for not acting adequately over concerns raised about Daniel.
Dr Pathan did not provide a "safety net" when teachers raised concerns over Daniel's appearance and weight. He was found guilty of misconduct in June but remains fit to practise.
A mother who starved and murdered her four-year-old son has died in jail.