RNA - ALQST, a UK-based rights group focusing on Saudi Arabia, confirmed to Al-Jazeera on Wednesday al-Fassi's arrest.
Considered a leading figure in women's rights in the region, and the kingdom, in particular, al-Fassi has long been fighting for the rights of Saudi women, including their right to participate in municipal elections.
As a scholar, her work focuses on women's history and politics. Al-Fassi was among the first Saudi women to drive for the first time since the religiously conservative country overturned the world's only ban on female drivers.
Last month, the government announced that a number of activists were being held for having suspicious contacts with foreign entities, as well as offering financial support to "foreign enemies".
Other suspects were being sought, the government said at the time, while state-linked media labelled those arrested as traitors and "agents of embassies".
Eight of the 17 detained activists, including five women, were later temporarily released "until the completion of their procedural review".
None of the activists has yet been officially charged, and they are being held incommunicado - with no access to their families or lawyers.
Earlier on Wednesday, United Nations experts urged Saudi Arabia to immediately release a number of women's human rights defenders arrested in the nationwide crackdown.
"In stark contrast with this celebrated moment of liberation for Saudi women, women's human rights defenders have been arrested and detained on a wide scale across the country, which is truly worrying and perhaps a better indication of the Government's approach to women's human rights," they said in a statement.
"We call for the urgent release of all of those detained while pursuing their legitimate activities in the promotion and protection of women's rights in Saudi Arabia," they added.
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