Ahmad Batebi was arrested in 1999 after being photographed holding up a blood-stained shirt of a friend during a student protest.
He was given a death sentence for endangering national security that was reduced to 10 years in prison on appeal.
He was later released on "unlimited" parole but was detained again in July, his lawyer Khalil Bahramian told Reuters on Saturday.
Batebi had threatened to go on hunger strike if he was taken back to prison but he did not carry out the threat, Bahramian said.
"He is not on a hunger strike but his wife has said that his condition is worrying, he had lost a lot of weight and he was being kept in the solitary prison," the lawyer said.
"[Batebi's] wife has been allowed to visit him twice and in the presence of a prison officer who asked them to only talk of family matters and nothing else," Bahramian said, adding that he was arrested in July "without a warrant or indictment".
He had been on parole for more than 18 months, Bahramian said. "If they have arrested him to have him complete his prison term, then why are they keeping him in solitary for the last 40 days which is an obvious torture?"
Iranian judiciary officials could not immediately be reached for comment.
In Washington, US State Department spokesman Sean McCormack voiced US concern on Friday about Batebi's arrest. He also said Washington was concerned about other Iranian student activists he said had recently been detained.