Mehdi Hashemi was accused of involvement in massive protests that followed Iran's disputed presidential election in 2009, and after being threatened with arrest he left for Britain.
The now 45-year-old was arrested after returning to Tehran in September 2012, and although initially bailed three months later he was rearrested and put on trial.
His conviction relates to national security matters as well as fraud and embezzlement, judiciary spokesman Gholamhossein Mohseni-Ejeie was quoted as saying on its official website and in state media.
Hashemi has 20 days to appeal his conviction, the reports said.
The 15-year jail term, if confirmed by a secondary court, would be one of the heaviest ever handed down to a family member of such a high-ranking official.
Hashemi supported the so-called Green Movement led by the defeated reformist candidates Mir-Hossein Mousavi and Mehdi Karroubi after the presidential election which was officially won by Mahmoud Ahmadinejad.
The former president's son, most commonly described as a businessman, actively supported Mousavi, dismissing Ahmadinejad's win as fraudulent.
Mousavi, along with his wife Zahra Rahnavard, and Karroubi were placed under house arrest in 2011 after repeatedly challenging the official election results, which gave Ahmadinejad a second term as president.
Mousavi and Karroubi are accused of "sedition" against the regime, and their most hardline opponents have said the two men should face the death penalty.
Rafsanjani, who was president from 1989 to 1997 and is now considered a moderate, is close to the reformist camp in Iranian politics.
In 2009, he became the bane of conservatives who have publicly echoed the doubts of some Iranians on the fairness of the election and criticised the repression and deadly crackdown by the regime that followed.
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