The actor said he had seen a motorist drive through a red light and crash into another car, which flipped onto its roof with a woman and baby inside.
The Machete star then rescued the baby with the help of a female bystander.
“She… undid the seatbelt and I pulled the baby out the other side,” Danny Trejo said.
In an interview about the incident shared on his Instagram page, the 75-year-old actor urged drivers to “pay attention” on the roads. He said he had initially tried to crawl through the window of the car himself, but was unable to get the baby out.
The female bystander, who identified herself as Monica Jackson, then came to his aid. Danny Trejo said she crawled into the vehicle and undid the seatbelt, allowing him to extricate the baby.
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The two said that the woman in the vehicle, who appeared to be injured, had called on them to save the child.
“She was hurt, but all she said was, ‘Get my baby. Get my baby,'” Ms Jackson said.
Los Angles Police Department confirmed to US media that the actor was at the scene of the accident.
Local TV station KABC-TV reported that no-one was seriously injured in the crash, but three people were taken to hospital for medical evaluation.
Danny Trejo has starred in numerous Hollywood films, often playing the role of the villain. He is perhaps best known for his role in Spy Kids and Machete.