The Revolutionary Court in Tehran sentenced 41-year-old Olivier Vandecasteele to 12.5 years in prison for espionage, 12.5 years for "cooperation with hostile governments", 12.5 years for money laundering and 2.5 years for currency smuggling.
He was also fined one million dollars, according to Iranian state media.
Belgium and Vandecasteele's family insist on the 41-year-old's innocence and say his detention is part of Tehran's efforts to get Brussels to release an Iranian diplomat convicted of terrorism.
The Iranian was sentenced in 2021 for supplying an Iranian-Belgian couple with explosives intended to be used against Iran's exiled opposition in Paris.
Human rights organizations have often accused Iran of using prisoners like Vandecasteele as pawns in negotiations with the West.
Belgium's call: Leave Iran
In the context of the ongoing nationwide protests in Iran, which started after the death of 22-year-old Mahsa Zhina Amini in the custody of the morality police, Belgium has urged its citizens to leave Iran due to the risk of arrest.
Close to 20,000 people are believed to have been detained since the demonstrations began in September.
Whether Vandecasteele's case is related to the protests is unclear.