Faleh’s wife, Jamila al-Ukalaa, told AFP she was informed by the security services in a telephone call at midnight (2100 GMT) on Monday that her husband had been arrested in Riyadh.
"I was told he is in the custody of the security services. They did not give me any reason," she said by telephone from the Saudi capital.
Ukalaa said she had gone to the university where Faleh teaches to look for him on Monday after he failed to come home and found only his car in the car park.
Faleh was among three prominent Saudi reformists who spent 17 months in jail for demanding a constitutional monarchy before being pardoned by King Abdullah in August 2005.
Mufleh al-Kahtani, vice president of the National Society for Human Rights, told AFP that the Saudi watchdog was "following up the case," but he did not know the basis for Faleh’s arrest.
In a statement received by AFP, an Arab rights group implicitly linked Faleh’s arrest to his defence of fellow reformist Abdullah al-Hamed, who is serving a six-month jail sentence on charges of inciting women’s protests.
Hamed was one of the three reformists jailed for advocating a constitutional monarchy and freed under a royal amnesty.
Hamed and his brother were sentenced in November to jail terms of six and four months on charges of inciting women to stage public protests, which are banned in Saudi Arabia.
Faleh, who is Hamed’s legal representative, recently "issued a statement about conditions in Buraida prison" after he visited his fellow activist in the jail, some 320 kilometres (200 miles) north of Riyadh, the Arab Committee for Human Rights said.
A number of reform advocates have been arrested over the past 15 months in Saudi Arabia, whose record on human rights is often criticised by international watchdogs.
The Gulf oil powerhouse, which is ruled by an absolute monarchy and has no elected parliament, denies violating human rights.
