The trial of Hathloul, who has been detained for more than two years, began in March 2019, but campaigners and her family say court sessions are arbitrarily announced and closed to diplomats and journalists.
"We were just announced that @LoujainHathloul has a trial tomorrow," the activist's sister Lina Hathloul wrote on Twitter on Tuesday.
A source close to the activist's family told AFP that judicial authorities did not specify a time for the hearing.
There was no immediate comment from authorities in Saudi Arabia, an absolute monarchy that has long faced international criticism for its human rights record.
"The only just outcome for this trial would be the immediate and unconditional release of Loujain al-Hathloul," said Lynn Maalouf, from Amnesty International.
"She is not a criminal - she is a human rights defender who is being punished simply for daring to advocate for change."
Hathloul, 31, was arrested in May 2018 with about a dozen other women activists just weeks before Saudi Arabia lifted its decades-long ban on female drivers.
Some have been provisionally released, but Hathloul and others remain in detention on what rights groups describe as opaque charges that include contact with foreign media, diplomats and activist groups.
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The pro-government Saudi media has branded Hathloul and others as "traitors", and her family alleges she experienced sexual harassment and torture in detention.
Saudi authorities vigorously deny the charges.
The detention of women activists has cast a spotlight on the human rights record of the kingdom, which has also faced intense global criticism over the 2018 murder of journalist Jamal Khashoggi in its Istanbul consulate.
Hathloul has been on a hunger strike in prison since October 26 to demand regular contact with her family, her siblings said, voicing concern over her frail health.
For months, they said, the activist has been permitted only limited contact with her family.
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