The actress was shopping in London's West End with boyfriend Dev Patel, her co-star in the film, when the fan ran towards them to say hello.
But Freida, 25, who was being followed by photographers, turned to have her picture taken and accidentally bumped into the woman.
The fan was sent flying into the middle of Regent Street, near Liberty, just as a car was approaching.
A witness said: 'As Freida turned around, she bumped into the fan because she didn't see her.'
Freida ran into the road and helped the woman out of the car's path, said the witness, adding: 'Afterwards, Freida made sure the woman was OK. It was quite a shock for all, especially Freida and the fan.'
Neither the fan nor the actress were hurt in the incident.
Freida is currently filming Oscar-winning director Woody Allen's as yet untitled new film with actor Josh Brolin.