Amir Javadifar, "a student of industrial management in Qazvin (city) died in prison," the reformist Etemad newspaper said, adding that his family has been asked to come for the body Sunday morning.
Etemad said Javadifar had been arrested in July 9 protests and had injuries in his arm and nose but it did not elaborate on the cause of death.
Newspapers reported on Saturday that Mohsen Ruholamini, 25, who had also been arrested on July 9 when thousands of protesters took to the streets on the anniversary of a bloody student uprising in 1999, also died in custody.
Hundreds of thousands of Iranians poured into Tehran streets after the disputed June 12 re-election of Ahmadinejad to protest their "stolen votes" before a crackdown by security forces.
Iranian official reports say at least 20 people died and more than 1,000 were arrested in the protests. Dozens of reformist leaders, journalists and human rights activists have also been jailed in the wake of the disputed election.
Iran's opposition leaders have warned against mistreatment of the detainees and called for their unconditional release.
