In the 9.15pm incident yesterday, Ong Ming Chiang, 26, was driving from Jalan Kuching to Selayang when her Honda Accord was hit from behind.
When she pulled over to check the damage, four men rushed out of their car and held her.
One of the carjackers then drove off in her car with her 14-month-old son in the back seat.
The other three men left Ong and fled in their car.
She stopped a taxi and used the driver's handphone to call her husband Ong Yong Ming, 27.
Both of them tried to call her mobile phone, which was still in her car, in an effort to inform the carjackers about the baby. However, there was no answer.
The couple later received a call from a man who found their son strapped to his baby seat outside a restaurant, about 2km from the crime scene.