Angela Radtke, from San Antonio, Texas, saw the one-year-old locked in a supermarket car park and raised the alarm.
Despite being warned by security guards that she could be arrested for damaging the car, she told witnesses: "I don't care if I get arrested, I'm going to save this baby."
She then smashed through the windscreen with a tyre iron. Angela then climbed inside the car to pull the baby out - cutting her arms, legs and back on broken glass in the process.
When she pulled the baby out he had a temperature and looked red.
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Angela, who wasn't charged because of Texas's 'Good Samaritan' act, said she didn't hesitate to spring to action.
She local TV news: "I felt like some people were kind of scared. They didn't want to be liable because the security guard was yelling that I could possibly be arrested for breaking the window."
CCTV cameras revealed that the child's father, 36-year-old Richard Elliott, was in the supermarket for nearly 40 minutes with his 12-year-old daughter, leaving the baby alone.
Staff tried to reach him over the store's tannoy, but he didn't respond.
When he finally emerged to find the commotion surrounding his car, he told authorities he had forgotten about the baby in the back seat.
The baby was taken to hospital and treated for dehydration before being placed in the custody of Child Protective Services.
A preliminary information report from the San Antonio Police Department stated: "The child appeared to be suffering from heat exposure and dehydration, but was not critically injured."
Mr Elliott has now been charged with child endangerment. The child's mother was at home at the time of the incident.
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