Hossein Mousavian was arrested in May last year and briefly detained in jail until he was released on bail.
Iran has released the news on a new holiday celebrating the first time the country enriched uranium.
As deputy head of Iran’s nuclear negotiating team, Mr Mousavian played a key role in talks with the West.
The BBC’s Frances Harrison, a former Tehran correspondent, says it was a shock when a regime insider like Mr Mousavian was taken from his home by eight security officials last year and questioned in Tehran’s notorious Evin Jail in connection with unspecified allegations of espionage.
Power struggle?
Months later, Iran’s hardline intelligence minister accused Mr Mousavian, a reformist, of having passed sensitive information to the British Embassy in Tehran.
Many saw the arrest of Mr Mousavian as part of a power struggle between the hardline government of President Mahmoud Ahmedinejad and moderates close to his rival, Hashemi Rafsanjani, our correspondent says.
Mr Mousavian was known to be close to Mr Rafsanjani.
Iranian news agencies report that Mr Mousavian has now been given a two-year suspended jail sentence.
He was found guilty of “breaching national security” on Tuesday, in a retrial.
Mr Mousavian had been acquitted of espionage charges but convicted of the lesser charge of “propaganda against the system” in an initial trial in November last year.
The official IRNA news agency said then court took into account Mr Mousavian’s track record of service and the fact that it was his first offence.
A lawyer for Mr Mousavian said he had been banned from holding diplomatic positions for the next five years but not from holding any government position.
