Samar Badawi is the sister of Saudi blogger Raif Badawi, who was lashed 50 times last year and is serving a 10-year prison sentence for insulting the kingdom's influential religious establishment.
She is also the wife of human rights lawyer Waleed Abulkhair, who is serving a 15-year prison sentence on charges related to his work, which included defending Raif Badawi.
Amnesty International and the Center for Inquiry say the arrest of Samar Badawi yesterday is believed to be linked to her alleged role in managing a Twitter account campaigning for the release of her husband.