The broadcaster did not identify the activist nor specify the date of his arrest. It said he had been picked up by intelligence agents in West Azerbaijan province, which borders Turkey.
Iran has been holding French teachers' union official Cecile Kohler and her partner Jacques Paris since May 7 on suspicion of "entering the country to sow chaos and destabilise society".
State television has aired footage of the couple from the moment they set foot in Iran on 28 April.
French trade union officials have said the couple were on holiday in Iran over the Easter school break.
The French government has condemned their arrest as "baseless" and demanded their immediate release.
Iran's schools have been hit by months of industrial action by teachers, with regular protests outside education ministry offices across the country.
On Wednesday, a group of independent UN rights experts expressed alarm over what they called a "violent crackdown" on teachers and wider civil society in Iran.
The experts, who are appointed by the UN but do not speak on its behalf, alleged that more than 80 teachers had been arrested or summoned by the authorities in the Islamic republic.
They also alleged five people had been killed in wider protests since the start of May, due to "excessive use of force by security forces".
Iran's economy has been reeling under punishing US sanctions imposed since 2018, when then president Donald Trump abandoned a nuclear deal between Iran and major powers.
Talks on reviving the agreement with the United States again a party have stalled since March in a blow to hopes of a boost to the economy from an easing of US sanctions.
Iran announced a series of measures to tackle the worsening economy in May, including changes to a subsidy system and increases in the prices of staple goods.
Hundreds protested in Iranian cities, including in Tehran province, state news agency IRNA reported.