Iran claims that as a former US Marine, Amir Mirzaei Hekmati received special training and served at US military bases in Iraq and Afghanistan before heading to Iran for his alleged intelligence mission.
The 28-year-old former military translator was born in Arizona and went to school in Michigan. His family is of Iranian origin. His father, a professor at a US community college, has said his son is not a CIA spy and was visiting his grandmothers in Iran when he was arrested.
Iran, which says it only seeks nuclear reactors for energy and research, has sharply increased its threats and military posturing against stronger pressures, including the US sanctions targeting Iran's Central Bank in attempts to complicate its ability to sell oil.
US State Department has demanded Mr Hekmati's release.
The court convicted him of working with a hostile country, belonging to the CIA and trying to accuse Iran of involvement in terrorism.
In its ruling, a branch of Tehran Revolutionary Court described Mr Hekmati as a mohareb, an Islamic term that means a fighter against God, and a mofsed, or one who spreads corruption on earth. Both terms appear frequently in Iranian court rulings.
In a closed court hearing in late December, the prosecution asked for the death sentence.
The US government has called on Iranian authorities to grant Swiss diplomats access to Mr Hekmati in prison. The Swiss represent US interests in Iran because the two countries do not have diplomatic relations.
Similar cases against Americans accused of spying have heightened tensions throughout years-long stand-off over Iran's nuclear program.