Alireza Jamshidi says Esha Momeni is being held in Tehran's Evin prison.
Momeni was arrrested in Tehran on October 15 while visiting from the United States, where she lives.
Activists say she was in Iran conducting research on the women's movement as part of her studies at California State University.
"The charge against her is crime against national security and her case is currently under preliminary investigation," Jamshidi is quoted by Reuters as saying.
He did not elaborate.
Activists say dozens of them have been detained since they launched a campaign in 2006 to collect 1 million signatures in support of demands to end what they see as legal discrimination of women in Iran.