Judiciary spokesman Gholamhossein Esmaili did not identify the man, but said he "had been free on bail".
Iranian media reported last week that a US-Iranian businessman named Emad Shargi had been arrested has he tried to cross a western border illegally.
It could complicate plans by new US President Joe Biden to re-engage diplomatically with Iran.
His predecessor Donald Trump subjected the Tehran government to a "maximum pressure" campaign of crippling economic sanctions.
Mr Trump wanted to compel Iranian leaders to renegotiate a 2015 nuclear deal that he abandoned, but they refused to do so and retaliated by violating a series of key commitments.
Mr Biden has said he is open to rejoining the accord and easing the sanctions if Iran returns to full compliance.
Iran has detained a number of US-Iranian dual citizens and Iranians with US permanent residency in recent years, most of them on spying charges.
At a news conference on Tuesday, Mr Esmaili responded to a question about the reported arrest of an US-Iranian "defendant".
He said Iranian law did not recognise dual nationality, but that "the defendant had been free on bail... and was arrested as he tried to leave the country". This person "faced charges from earlier in the area of spying and gathering information for foreign countries", he added.
Earlier this month, Iran's Young Journalists' Club (YJC) news agency reported that Emad Shargi, who it said worked at an Iranian venture capital company called Sarava Holding, had been arrested on after "trying to illegally flee the country from western borders".