The Avatar star, 33, rushed to the aid of an elderly woman who had reportedly gotten into a car accident in Culver City, Calif., on Wednesday.
"Zoe witnessed the car accident, and was very quick to help," says an onlooker. She "immediately got on the phone with 911 and she seemed very concerned."
Saldana and another passerby transported the woman – who was conscious but injured – from her car to the curb. There they called 911 and waited for police and paramedic attention on the scene.
The onlooker adds: "While waiting for help to arrive, Zoe returned to the woman's car and picked up the woman's handbag and sweater. Zoe was very caring and sweet to the woman."
A rep for Saldana had no comment.