Mohsen Shekari was hanged on Thursday morning after being found guilty by a Revolutionary Court of “enmity against God”, state media reported.
He was accused of being a “rioter” who blocked a main road in Tehran in September and wounded a member of a paramilitary force with a machete.
An activist said he was convicted after a “show trial without any due process”.
Mahmood Amiry-Moghaddam, director of Norway-based Iran Human Rights, tweeted that executions of protesters would start to take place daily unless Iranian authorities faced “rapid practical consequences internationally”.
The judiciary’s Mizan news agency reported that a Revolutionary Court was told that Mohsen Shekari had blocked Tehran’s Sattar Khan Street on 25 September and used a machete to attack a member of the Basij Resistance Force, a volunteer paramilitary force often deployed to quell protests.
On 1 November, the court found Shekari guilty of fighting and drawing a weapon “with the intention of killing, causing terror and disturbing the order and security of society” and convicted him of “enmity against God”, Mizan said.
He appealed against the verdict, but it was upheld by the supreme court on 20 November, it added.
The judiciary has so far announced that 10 other people have been sentenced to death by Revolutionary Courts on the charges of “enmity against God” or “corruption on Earth” in connection with the protests. The defendants’ identities have not been disclosed.
Amnesty International said the death sentences were designed to “further repress the popular uprising” and “instil fear among the public”.
Revolutionary Courts operated “under the influence of security and intelligence forces to impose harsh sentences following grossly unfair trials marked by summary and predominantly secret processes”, it added.
The protests against Iran’s clerical establishment erupted in mid-September, after the death in custody of Mahsa Amini, a 22-year-old woman who was detained by morality police for allegedly wearing her hijab, or headscarf, “improperly”.
The women-led protests have spread to 160 cities in all 31 of the country’s provinces and are seen as one of the most serious challenges to the Islamic Republic since the 1979 revolution.
Iran’s leaders have portrayed them as “riots” instigated by the country’s foreign enemies and ordered security forces to “deal decisively” with them.
So far, at least 475 protesters have been killed and 18,240 have been detained, according to the Human Rights Activists’ News Agency (HRANA). It has also reported the deaths of 61 security personnel.