"We are aware that a Swedish citizen, a man in his 30s, has recently been detained in Iran. The embassy in Tehran is seeking information on the case and is in contact with local authorities," the Swedish Foreign ministry said in a statement to Reuters.
The ministry offered no further details but Swedish newspaper Aftonbladet reported the man was on holiday with other Swedes when he was detained.
There was no report of any such arrest on Iran's media.
Sweden advised citizens against non-essential travel to Iran on April 28 as the trial of Hamid Noury, 61, was drawing to a close.
The Iranian former prison official is accused of having played a leading role in the executions of political prisoners on government orders at the Gohardasht prison in Karaj, Iran, in 1988. He denied the charges.
He was arrested at a Stockholm airport in November 2019 after Iranian dissidents in Sweden filed police complaints against him. His trial ended on Wednesday, with the court due to issue a verdict in July.
Iran's foreign ministry summoned the Swedish envoy on Monday to protest against "the baseless and fabricated accusations that the Swedish prosecutor made against Iran during Noury's court case", Iranian media reported.
Two days later, Iran's semi-official Isna news agency said a Swedish-Iranian national sentenced to death in Iran on charges of spying for Israel will be executed by May 21. Ahmadreza Djalali, a disaster medicine doctor and researcher, was arrested in 2016 while on an academic visit to Iran.
Iran's powerful Revolutionary Guards have arrested dozens of dual nationals and foreigners in recent years, mostly on espionage charges. Rights activists have accused Iran of using them as bargaining chips. Iran, which does not recognise dual nationality, denies taking prisoners to gain diplomatic leverage.
However, Iran has exchanged several jailed foreigners and dual nationals with Iranians detained abroad.
With reporting from agencies.