The woman, identified only as Shema, had been protesting against the ban on woman drivers in the Gulf kingdom. She was found guilty of driving in Jeddah last July.
Women2drive, which campaigns for women to be allowed to drive in Saudi Arabia, says she has already lodged an appeal.
In recent months, scores of women have driven vehicles in Saudi cities in an effort to put pressure on the monarchy to change the law.
The sentence comes two days after the Saudi leader King Abdullah announced women would be allowed to vote for the first time in 2015.
Two other women are due to appear in court later this year on similar charges, correspondents say.
Newswarped wonders how Danika Patrick would get on if she was competeing in Saudi Arabia. I guess the leaders would get in more of a tizz over some of her publicity shots than the fact she is driving.