Chinese-American Xiyue Wang was imprisoned in July for “collaborating with foreign governments” – a charge he denies.
He was arrested while doing research in Iran for a university dissertation.
Three others facing similar charges have also had their jail sentences upheld. Two have joint US-Iranian citizenship, and one is Lebanese.
The court rulings come at a time of rising tension between Iran and the US.
Washington has demanded the immediate release of the Americans, who it said were being held on fabricated national security charges.
The US State Department accused Iran of “harassing, arresting and detaining US citizens”.
People of dual nationality were being especially singled out, it said, while the ability of the US to help those being held was “extremely limited”.
Mr Wang was arrested in August 2016 as he tried to leave the country, according to the Iranian judiciary’s news agency.
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He was accused of seeking to gather “highly confidential articles” for US and British academic institutions.
“Before his arrest he was able to digitally archive 4,500 pages of the country’s documents while under covert surveillance,” the agency said.
Earlier this year, the authorities in Tehran said as many as 70 people they described as spies were serving sentences in the capital’s jails.
The identities of only a handful have been made public.
President Donald Trump has warned Iran that it will face serious consequences if the American prisoners are not immediately released.
That it turn led to Iran accusing the US of wrongfully imprisoning Iranian citizens.
Iran’s Foreign Minister Mohammad Javad Zarif accused Washington of holding Iranians on “charges of sanction violations that are not applicable today… for bogus and purely political reasons”.
In January 2016, the two countries conducted a groundbreaking prisoner swap by which Iranians held or charged in the US – mainly for breaching sanctions regulations – were released in exchange for Americans jailed in Iran.
The exchange took place days after international sanctions on Iran were liftedas part of a deal designed to prevent it developing nuclear weapons.
As carried in bbc on 5.9.2017