Iran on Friday freed a Belgian aid worker, Olivier Vandecasteele, after almost 15 months in custody, in a prisoner exchange for an Iranian diplomat convicted of terrorism.

Belgium's Prime Minister Alexander De Croo said the 42-year-old had arrived in Oman and added: "If all goes to plan, he'll be with us this evening. Free at last!"

Separately, Oman's foreign ministry announced it had helped broker an "exchange deal" and that an Iranian previously held in Belgium was on his way to Tehran.

Iran announced that the freed Iranian is diplomat Assadollah Assadi, who was jailed in Belgium over a 2018 plot to bomb an Iranian opposition rally outside Paris.

Iran's Foreign Minister, Hossein Amir-Abdollahian, tweeted that Assadi, "the innocent diplomat of our country... is now on his way back to his homeland and will soon enter our beloved Iran."

Both Belgium and Iran thanked Oman for its role in securing the release.

Belgium has always insisted that Vandecasteele was innocent and his trial rigged.

He was sentenced in January to 40 years in prison and 74 lashes for "espionage", Tehran's judiciary said at the time.

"Olivier spent 455 days in prison in Tehran in unbearable conditions. Innocent," De Croo said.

"For me, the choice has always been clear. Olivier's life has always come first. It's a responsibility that I take upon myself, that I accept. In Belgium, we do not abandon anyone."

Last year, Belgium and Iran signed a treaty to enable prisoner swap. (AFP)