The Foreign Ministry, in a statement carried by the official IRNA news agency late today, said the journalist defected last week and was suspended from his job at the semi-official ISCA agency.
ISCA is a less known media outlet in Tehran.
The ministry did not identify the reporter but ISCA's editor Amir Hossein Motaghi was covering the talks in the Swiss town of Lausanne.
Earlier, Motaghi had worked as a sports reporter and was also one of scores of Iranian journalists active in the media section of President Hassan Rouhani's campaign headquarters in the run-up to Iran's 2013 presidential election.