Saudi Arabia is the only country in the world which does not allow women to drive.
Loujain Hathloul and Maysaa Alamoudi "have been detained for 25 further days" the London-based watchdog said in a statement.
"Jailing a woman for simply driving a car is preposterous," Said Boumedouha, Amnesty's deputy director for the Middle East and North Africa, was quoted saying in the statement.
The Interior Ministry has still not commented on the case of the two women.
Border officers stopped Hathloul when she tried to drive from neighbouring United Arab Emirates into Saudi Arabia on November 30.
Alamoudi, a UAE-based Saudi journalist, later arrived to support her.
They were arrested and are being held in Saudi Arabia's Eastern Province.
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Relatives of the two women declined to comment on Tuesday.
Hathloul was trying to make a point in her unusual attempt to drive through the border and knew that she would not be allowed to pass, an activist has told AFP.
Women drivers in the kingdom have previously been arrested and cars have been confiscated but the detention of Hathloul is already among the longest given to any female driver in the kingdom recently, activists told AFP.
In 2011, Manal al-Sharif was held for 10 days after posting a video of herself driving in the eastern city of Khobar.
Activists have started a petition calling for the release of Hathloul and Alamoudi.
During October dozens of women drove in the kingdom and posted images of themselves doing so as part of an online campaign supporting the right to drive.
In response, the Interior Ministry said it would "strictly implement" measures against anyone undermining "the social cohesion".
Activists say it is not actually against the law for women to drive and that the ban is linked to tradition and custom in the kingdom.