File photo of protesters calling on the United Nations to take action against the treatment of women in Iran following the death of Mahsa Amini. AFP
"Javad Rouhi, an inmate in Nowshahr city prison, was transferred to Shahid Beheshti Hospital in the city early Thursday after suffering a seizure while in prison," the judiciary's Mizan Online news website reported.
"Unfortunately, he died despite the actions of medical staff, and a legal case has been filed to follow up on the reason for his death," it added.
Rouhi, whose original death sentence had been overturned by the Supreme Court, died almost a year after a nationwide protest movement was triggered by the September 16 death in custody of Iranian Kurd Amini.
The 22-year-old had been detained for allegedly breaching the Islamic republic's strict dress code for women.
During months of protest, which Tehran called foreign-instigated "riots", thousands of Iranians were arrested and hundreds killed, including dozens of security personnel.
Seven men have been executed in cases related to the protests that involved killings and violence against members of the security forces.
Rouhi was sentenced to death on the charge of "corruption on Earth" in Nowshahr in the northern province of Mazandaran for "leading a group of rioters", "inciting people to create insecurity", and "apostasy by desecration of the Koran by burning it", Mizan said in January.
The then 31-year-old was also found guilty of "setting fire to and destroying property in a way that causes severe disruption to the country's public order and security", it added.
However, the Supreme Court overturned his death sentence in May and referred his case to another court for re-evaluation.
In January, Amnesty International said that Rouhi had been subjected to physical torture.
The London-based rights group says Iran executes more people than any other country except China and hanged at least 582 people last year, the highest number since 2015.