Iran's judiciary confirmed on September 12 that it was holding a Swedish national, saying he had "committed crimes" in the Islamic republic.
"This Swedish citizen has committed crimes in the territory of the Islamic Republic of Iran," judiciary spokesman Massoud Setayeshi said.
Last week, the European Union said Swedish diplomat Johan Floderus, a 33-year-old who works for the bloc, had been detained in Iran for more than 500 days.
Mr. Setayeshi said the Swede was sent "to prison with a legal order".
He said an investigation into the case "is being finalised" and will be sent to the relevant court "in the coming days".
The EU's top diplomat Josep Borrell said last week that the bloc had been pushing "relentlessly" for the Swede's release.
The Swedish government has also called on Iran to free Floderus.
"We have noted the statement by the Iranian authorities," the press office of Sweden's ministry of foreign affairs said Tuesday.
"Johan Floderus is arbitrarily detained and should therefore be released immediately," it stressed, adding that "this has been conveyed to the Iranian authorities."
Iran announced in July last year that it had arrested a Swede on suspicion of espionage.
The announcement came two weeks after an Iranian, Hamid Noury, was sentenced to life in prison in Sweden on charges related to killings of prisoners in Iran during the 1980s.
A Stockholm court had found Noury guilty of "aggravated crimes against international law" and "murder".
Mr. Setayeshi said the case of two French nationals in custody on espionage charges had already been referred to court for trial.
Cecile Kohler and her partner Jacques Paris were arrested in May 2022 over accusations of seeking to stir labour unrest during industrial action by teachers.
In May this year, Belgian humanitarian worker Olivier Vandecasteele, 42, was released after 15 months' custody in Iran on espionage charges.
Mr. Vandecasteele's release took place in exchange for Iranian diplomat Assadollah Assadi, who was sentenced to 20 years for plotting to bomb an opposition rally in Paris.