Zeinab Taheri was taken into custody after an arrest warrant was issued against her for making "false statements" and because of "her lies propagated online claiming (the defendant) was not guilty," the conservative-linked agency said.
It did not specify the date of her arrest.
"The female lawyer who recently claimed to be the defendant's lawyer and who has gone hoarse saying he is innocent was basically not this person's lawyer," Tehran's chief justice Gholamhossein Esmaili said, according to the agency.
Mohammad Reza Salas was hanged on Monday for driving a bus into a group of police officers during February clashes with security forces, killing three of them.
Salas belongs to the minority Sufi sect, a mystic branch of Islam tolerated in Iran but perceived as a "deviation" by many conservatives members of the Shiite community.
Taheri's name appears in a statement released on Monday by Human Rights Watch, which presents her as Salas's lawyer.
The statement, which addresses "serious allegations of torture to force confessions", refers to an interview Taheri gave with another foreign-based NGO in which the lawyer claims that Salas was violently beaten in prison to the extent he nearly lost his eyesight.
Taheri also represented Iranian scholar Ahmadreza Djalili, sentenced to death for spying for Israel in a trial rights group Amnesty International called a "secret and hasty procedure that did not allow arguments from the defence".
On the day of Salas's execution, posts from a Twitter account bearing Taheri's name said it would "reveal for public opinion all possible" evidence of his innocence -- later stating it would not do so over requests from the family.
The same account posted a tweet in favour of Salas's "innocence" today, after Taheri had been reportedly arrested.
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)