Siamak Namazi was arrested in October 2015 and charged with trying to overthrow the state, which he denies.
An Iranian American imprisoned in Iran has started a seven-day hunger strike and appealed to United States President Joe Biden to bring him home.
Siamak Namazi made the plea on Monday in a letter to Biden on the seventh anniversary of the release of five US citizens in a prisoner exchange with Iran
His father Baquer, who travelled to Iran in 2016 out of concern for his detained son, had also been arrested on espionage-related charges and released two years later but banned from travelling.
Namazi said in his letter he would be on a hunger strike for seven days, asking Biden to spend one minute a day for the next week thinking about the suffering of US citizens detained in Iran.
They include environmentalist Morad Tahbaz, 67, who also has British nationality, and businessman Emad Shargi, 58.