A Swedish citizen working for the European Union diplomatic corps has been imprisoned in Iran for more than 500 days, making him an important bargaining chip for Tehran as it tries to wring concessions from the West.

The arrest, which has been kept under wraps for over a year by the Swedish and European Union authorities, appears to be part of an expanding pattern of what has become known as Iran’s “hostage diplomacy.”

Tehran has been opportunistically scooping up dual Iranian nationals and foreigners on spurious charges, seeking to trade them for Iranians held in Europe or the United States, or to use them as leverage to extract money and other concessions.

Last month the United States concluded a deal with Iran to free five Americans held there in exchange for $6 billion in withheld Iranian oil revenues as well as the release of Iranian prisoners in America.

Still, this latest case, the details of which have not been previously reported, stands out for the prisoner’s professional background as a European official. The man, Johan Floderus, 33, a native of Sweden, has held several positions in the European Union’s institutions, coming up through its civil service traineeship program. He was even featured in an advertising campaign to attract young Swedes to European Union careers.

Mr. Floderus visited Iran last spring on what people familiar with the case described as a private tourist trip, together with several Swedish friends. As he prepared to take his flight out of Tehran on April 17, 2022, he was detained at the airport.

In July of last year, the Iranian government released a statement announcing that it had apprehended a Swedish national for espionage. He is now being held in the notorious Evin prison in the Iranian capital.

The New York Times spoke to six people with firsthand knowledge of the case. All requested anonymity, fearing a backlash for speaking about it. They denied that Mr. Floderus had been involved in espionage.

The Swedish Ministry for Foreign Affairs said it would not comment on the details of the case, citing a need for secrecy. “A Swedish citizen — a man in his 30s — was detained in Iran in April 2022,” its press department said in an email. “The Ministry for Foreign Affairs and the Embassy of Sweden in Tehran are working on the case intensively.”

“We understand that there is interest in this matter, but in our assessment it would complicate the handling of the case if the ministry were to publicly discuss its actions,” it added.

Mr. Floderus most recently served as an aide to the European commissioner for migration, Ylva Johansson, starting in 2019. In 2021, he joined the European External Action Service, the bloc’s diplomatic corps.

He had visited Iran previously, without incident, while on official European Union business, when he worked for the bloc’s development program, people familiar with his background said.

The Iranian statement announcing the arrest of a Swedish national in 2022 made note that the person had visited the country before, citing those visits as evidence of nefarious activity.

The European External Action Service said that it was “following very closely the case of a Swedish national detained in Iran,” but did not acknowledge that the person in question worked for the service or that Mr. Floderus had previously visited Iran on official E.U. business.

“This case has also to be seen in the context of the growing number of arbitrary detentions involving E.U. citizens,” added Nabila Massrali, a spokeswoman for the bloc’s diplomatic body. “We have used and will continue to use every opportunity to raise the issue with the Iranian authorities to obtain the release of all arbitrarily detained E.U. citizens.”

Reached by telephone, Mr. Floderus’s father declined to comment.

Mr. Floderus was a member of the Afghanistan delegation for the diplomatic corps, but never made it to Kabul because of the Taliban takeover in August 2021. He did his job from headquarters in Brussels, where he had lived for several years, people familiar with his background said.

“This arrest in 2022 was a real escalation,” said Richard Ratcliffe, the husband of Nazanin Zaghari-Ratcliffe, a British-Iranian charity worker who was held in Iran for six years on false charges of espionage. “It is shocking for me that the Swedish government and the E.E.A.S. have sat on it.”

Ms. Zaghari-Ratcliffe was released last year in exchange for Britain’s settling a longstanding financial debt with Iran.

A Belgian aid worker, Olivier Vandecasteele, who was similarly imprisoned in Tehran on espionage charges for 455 days, recently appeared to pay homage to Mr. Floderus without naming him.

After Mr. Vandecasteele was freed in a prisoner swap in May, he referred to a Swedish cellmate in the Evin prison at a concert held in his honor in Brussels in June.

“We became like brothers,” Mr. Vandecasteele said at the time. “We promised each other that we would do everything for each other and whoever came out first would help each other’s family and loved ones.”

Relations between Iran and Sweden are at a nadir. In July last year, a Swedish court sentenced a former senior Iranian judicial official, Hamid Noury, to life in prison over war crimes committed in 1988 in Iran. He is appealing.

The landmark case against Mr. Noury, who was found to have played a key role in the execution of thousands of Iranians, was a rare example of “universal jurisdiction,” under which countries can arrest foreign nationals on their soil and prosecute them for atrocities, irrespective of where the crimes were committed.

Just before Mr. Noury’s conviction in July 2022, Iran began escalating pressure on Sweden.

Mr. Floderus was arrested in April 2022. That May, Iran said it planned to execute an Iranian-Swedish scientist, Ahmadreza Djalali, on murky charges of spying and aiding Israel in assassinating nuclear scientists, accusations that he denies.

That same month, Iran also executed another Swedish-Iranian, the dissident Habib Chaab, who had been living in Sweden for more than a decade and was abducted during a visit to Turkey in 2020 and smuggled to Iran.

“My view is that the European governments keeping their new hostage cases quiet last year inevitably led to other escalations by Iran,” Mr. Ratcliffe said. “It is not a coincidence that they then started executing foreign nationals. Hostage diplomacy has shifted into execution diplomacy.”

Governments handling negotiations with the Iranian authorities often push for secrecy while they work out what to do, in part to avoid public scrutiny and pressures. Critics say the secrecy also allows them to pursue other policy priorities in talks with Iran without being held accountable.

“In our family’s experience, publicity keeps hostages safe because it limits the abuse that gets done to them, and it alerts everyone to the games being played,” Mr. Ratcliffe said.“When Western governments try to suppress these cases and keep families quiet, they are prioritizing other agendas than the welfare of their citizens,” he said.

The European Union is pursuing talks to revitalize a nuclear deal with Iran, with a goal of limiting Tehran’s advances in enriching uranium to a level very close to bomb grade.

Despite Western efforts to isolate Iran through sanctions and Tehran’s continued policy of arresting Westerners and executing and imprisoning activists at home, Tehran’s isolation has been increasingly pierced.

Last month, Iran was invited to join BRICS, the club of major developing-world powers led by China and Russia. It has also been helping Russia fight its war in Ukraine by providing it with armed drones, among other things.

Christina Anderson contributed reporting from Stockholm, Steven Erlanger from Berlin and Monika Pronczuk from Brussels.