Ayat al-Qurmazi was arrested in 2011 and said she was held in custody for nine days. She says she was abused by Noura Bint Ebrahim al-Khalifa, an officer in the country's drugs control unit.
Qurmazi's lawyer, Reem Khalaf, told the BBC her client saw the Bahraini princess present when she was abused while in detention.
"Noura was very angry that Ayat had seen her," the lawyer was quoted as saying.
Hundreds of activists were rounded up in 2011 when the Sunni-led kingdom was in the grips of the Arab Spring. The government there was accused of taking a heavy hand against its opponents.
This week, the monarchy said it was pressing again for a national dialogue to address national grievances.
An independent commission of inquiry examining the government's response to the unrest spelled out a series of reforms for the monarchy. Last year, however, the kingdom was criticized for placing a temporary ban on demonstrations and for revoking the citizenship from several opposition leaders.
The Bahraini princess is expected to appear before a court next week.