Samar Badawi was detained for allegedly managing a Twitter account calling for the release of her husband.
Amnesty International called the arrest "the latest example of Saudi Arabia's utter contempt" for human rights.
Abu al-Khair was jailed for 15 years for "undermining the regime" in 2014. He is the founder of the Monitor of Human Rights in Saudi Arabia group.
Mrs Badawi is also the sister of imprisoned Saudi blogger Raif Badawi, who in 2014 was sentenced to 1,000 lashes and 10 years in prison for insulting Islam.
Raif Badawi's wife, Ensaf Haidar, also reported the arrest in her tweet: "Urgent: #Samar_Badawi was arrested on the charge of directing @WaleedAbulkhair twitter account."
Samar Badawi was arrested on Tuesday in Jeddah and transferred along with her two-year-old daughter to a police station, Amnesty International USA quoted local activists as saying.
After questioning, she was sent to prison and is expected to appear before a prosecutor later on Wednesday, it says.
Amnesty's Philip Luther said the arrest "demonstrates the extreme lengths to which the authorities are prepared to go in their relentless campaign to harass and intimidate human rights defenders into silent submission".
Saudi officials have not publicly commented on the issue.
The wife of jailed prominent Saudi human rights campaigner Waleed Abu al-Khair has been arrested, activists say.