"We will not stop" until Johan Floderus is freed, Josep Borell said on Tuesday, confirming the identity of the Swedish national held prisoner in Iran.
The 33-year-old Swedish citizen is an employee of the European Union institutions, the bloc's foreign policy chief said.
Johan Floderus was detained in April 2022 by Iranian authorities and has been held for more than 500 days in the Evin prison, in Teheran.
The details of the case were first revealed on Monday morning by the New York Times, which reported Floderus was a member of the European External Action Service (EEAS), the EU's diplomatic service.
In its initial reaction, the European Commission refused to identify the person, simply indicating the case involved a Swedish national who was being used as a "pawn for political reasons" by the Iranian government.
But on Tuesday, Josep Borrell, who oversees the EEAS, spoke on the record and confirmed the information published by the newspaper, saying Floderus had been "illegally detained."
"I want to stress that, I personally, all my team, at all levels, the European institutions in close coordination with the Swedish authorities, which have the first responsibility of consular protection, and with his family, have been pushing Iranian authorities to release him," Borell told journalist while attending a ministerial meeting in Cádiz, Spain.
"Every time we had a diplomatic meeting, at all levels, we had put the issue on the table. Relentlessly, we have been working for the freedom of Mr Floderus. And we will continue doing that in close contact with the family, respecting their will, and for sure with the Swedish government."
"This is very much in our agenda, in our heart, and we will not stop until Floderus (is) freed," he added.
According to the New York Times, Floderus had previously visited Iran in his capacity as an EU official but was on a private tourist trip with friends the day he was detained.
Floderus was then charged with "espionage," an accusation that people familiar with the Swede strongly denied.
In a statement to the newspaper, the family said: "We, Johan's family, are deeply worried and in despair. Johan was suddenly and without reason deprived of his liberty on a vacation trip and has been in an Iranian prison for more than 500 days."
The New York Times report follows a series of episodes in which Iran has imprisoned dual citizens on widely contested criminal charges and agreed to release them only after obtaining certain concessions from Western governments.
In late May, Olivier Vandecasteele, a Belgian humanitarian worker who had been sentenced to 40 years in a Teheran prison, was released as part of a prisoner swap between Belgium and Iran. The United Nations described Vandecasteele's detention and treatment as a "flagrant violation of international law."
This article has been updated with new information.