Akbar Mohammadi, who was arrested for taking part in pro-democracy demonstrations in 1999, was the first political dissident known to have died in prison in Iran for many years.
"He was under intense supervision by the prison physician. Last night his health condition deteriorated," Jamal Karimirad said on Monday.
"He insisted on going back to his cell. His condition weakened again there and he died on his way to the hospital."
Another dissident student, who asked not to be named, said that Mohammadi had started his hunger strike in May.
However, Karimirad said it was not clear whether Mohammadi died because of his hunger strike.
"The coroner's office is investigating the reason for Mohammadi's death," he said.
Mohammadi was originally sentenced to death as a "mohareb", or "one who wages war against God", for his part in the protests, in which tens of thousands of people took to the streets after police and vigilantes attacked a peaceful campus rally in support of press freedoms.
But like other student activists sentenced to death, his punishment was reduced to a 15-year jail term in 2000 after the intervention of Ayatollah Ali Khamenei.
Khalil Bahramian, Mohammadi's lawyer, criticised the judiciary for not allowing him to visit his client in Tehran's notorious Evin prison, where most dissidents are held.
"When I heard about his hunger strike, I wanted to visit him. But I was denied by the prison authorities," he told local Iranian newspapers.
"This is a violation of international conventions and Iran's Islamic laws."