The 50-year-old woman had been imprisoned in a room in a house in the western Taif province, an official from the kingdom's National Society for Human Rights told AFP.
According to local daily Okaz, Saudi security forces freed the woman at dawn yesterday in the presence of representatives of the organisation.
The woman's son had alerted authorities that his mother was being held against her will due to a "family dispute over properties", the daily reported.
Ultra-conservative Saudi Arabia, which applies a strict version of Islamic sharia law, imposes many restrictions on women, who are banned from travelling without permission from male guardians and from driving.