"Yes, Loujain is free," said the campaigner who spoke with Loujain Hathloul after she left prison.
Hathloul "just said that she's released and she's happy," said the activist, who did not give a name.
Maysaa Alamoudi, detained at the same time as Hathloul, has also been let out of jail, her family confirmed, according to the activist who spoke with AFP.
"Peace be upon you, good people," Hathloul tweeted late on Thursday.
She and Alamoudi had been held since December 1, after Hathloul tried to drive into the kingdom from neighbouring United Arab Emirates in defiance of the ban.
Saudi Arabia is the only country in the world which does not allow women to drive.
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Alamoudi, a UAE-based Saudi journalist, arrived at the border to support Hathloul and was also arrested.
In December, activists said a court in Eastern Province had transferred the two women to a special tribunal for "terrorism" cases.
At the time, campaigners did not provide full details of the allegations against the pair but said investigations appeared to focus on the women's social media activities rather than the driving.
The activist who spoke to AFP on Friday did not know whether the two women were facing charges or what conditions were placed on their release.
Hathloul has 232,000 followers on Twitter.
Before her arrest she tweeted, sometimes with humour, details of the 24 hours she spent waiting to cross into Saudi Arabia after border officers stopped her.
Alamoudi has 136,000 followers and has also hosted a programme on YouTube discussing the driving ban.