FILE PHOTO: Freed Belgian aid worker Olivier Vandecasteele (L) embraces family members as he disembarks an aircraft on the tarmac upon his return to Belgium after almost 15 months of captivity in Iran, at Melsbroek military airport north of Brussels, on May 26, 2023. AFP
The governments of Denmark and Austria had been notified, he said.
The release of the three, as well as that of a Belgian aid worker a week earlier, were part of a prisoner swap in which Tehran got back an Iranian diplomat who has been convicted and incarcerated in Belgium on terrorism charges.
Vienna reacted with relief at the release of its two citizens, named as Kamran Ghaderi and Massud Mossaheb, who it said had been arrested "unjustly" by Iran in January 2016 and January 2019, respectively.
Thanking Belgium, Austrian Foreign Minister Alexander Schallenberg said: "Our years of diplomatic efforts to secure their release have borne fruit... Today is a very emotional day for all of us."
The Danish man, who was not immediately identified, had been arrested in Iran in November 2022 on the sidelines of a demonstration for women's rights, De Croo said in a statement.
"Belgium is currently organising their evacuation via Oman to Belgium" after medical examinations, he added.
They were expected to land around 3am (0100 GMT) Saturday at the Melsbroek military airport just outside Brussels, where Belgian Foreign Minister Hadja Lahbib would greet them, officials said.
That was the same airport where Belgian aid worker Olivier Vandecasteele arrived on May 26 upon being freed by Iran after 15 months in captivity.
His liberation was obtained in exchange for Belgium freeing Iranian diplomat Assadollah Assadi, who had been imprisoned for a 2018 plot to bomb an Iranian opposition rally outside Paris.
Iran had levelled charges of "espionage" at Vandecasteele but his family, the Belgian government and rights groups all say that was a fabricated case used to pressure Brussels for Assadi's release.
Belgian government officials said the release of Vandecasteele, the Dane and the two Austrian-Iranians were all part of "Operation Blackstone", in reference to an 18th-century English jurist, William Blackstone, who was known for declaring: "It is better that 10 guilty escape than one innocent suffer."
The exiled Iranian opposition group the National Council of Resistance in Iran, the target of the 2018 bomb plot, has criticised Assadi's release, saying it violated a Belgian court order requiring them to be consulted first.
Critics of the exchange said it would encourage Tehran to take more Europeans hostage as bargaining chips to seek the return of agents like Assadi arrested for terror offences in the West.
De Croo stressed his government "continues to fight for the respect of human rights and the release of European citizens unjustly detained by Iran".
There was no immediate confirmation from Iran of the three Europeans' release.
Dozens still held
The exact number of foreign passport holders held by Iran is thought to be in the dozens but is not precisely known, as the families of some detainees opt to negotiate out of the public eye.
Belgian government officials said at least 22 "innocent" Europeans remained detained in Iran. France last week gave a figure of more than 30 EU citizens held.
Austria's Schallenberg said of his two freed compatriots: "We are especially happy for the brave families who have suffered so much in recent years. Now they can finally embrace their husbands, fathers and grandfather again in freedom."
The Gulf sultanate of Oman has emerged as a key interlocutor between the West and Iran.
In 2016 it also played a mediator role in the release of Washington Post journalist Jason Rezaian and three other US citizens who had been held by Tehran.
In May, Iran released a Frenchman and a French-Irish citizen, both of whom had gone on hunger strike to protest their detention and conditions.