Ensaf Haidar, president of the Raif Badawi Foundation, confirmed her arrest to CNN.
Haidar said Badawi is charged with running the Twitter account of Waleed Abu al-Khair, a prominent human rights lawyer and her former spouse.
Al-Khair is serving a 15-year prison sentence, which Amnesty International says is "in connection with his work protecting and defending human rights in Saudi Arabia."
His Twitter account
has been used recently as a platform for those campaigning for his release.
"Words are not enough for me to express how proud I am of my husband," Badawi wrote in an open letter published the human rights group in December.
"To all those rulers and judges who have unfairly imprisoned the free, and enslaved the people, beware of the judgement you will receive from the heavens above. Woe to you who have terrorized the aggrieved out of pride."
Amnesty International called Badawi's arrest
"the latest example of Saudi Arabia's utter contempt for its human rights obligations and provides further damning proof of the authorities' intent to suppress all signs of peaceful dissent."
Badawi is set to appear before a prosecutor on Wednesday, according to Amnesty.
CNN has reached out to the Saudi government, which has not yet responded to a request for a statement.
Badawi served seven months in jail in 2010 for disobeying her father, who she said physically abused her from the age of 14 after her mother died of cancer.
"I was very hurt when I went to prison," she told CNN in 2012
. But I came out victorious and was very proud of myself that I was able to handle those seven months. It wasn't easy."
First lady Michelle Obama and then-U.S. Secretary of State Hillary Clinton presented her
with an International Women of Courage Award in 2012.
Her brother, Raif Badawi, is imprisoned in Saudi Arabia. He was convicted
of violating Saudi Arabia's information technology law and insulting Islam on his blog, "Saudi Arabian Liberals."
His case drew international attention after he was sentenced to 10 years in prison and 1,000 lashes.
That sentence was upheld after an appeal last year.