Pamela Rauseo, 37, of West Kendall, stopped her vehicle in the middle of westbound traffic Thursday, screaming out for help for her nephew, Sebastian de la Cruz, the Miami Herald reported.
The baby, who was born premature and has respiratory problems, wasn't breathing and was turning blue, witnesses said.
Drivers exited their cars to help Rauseo, who had taken the baby out of her vehicle and was performing CPR.
Sweetwater police officer Amauris Bastidas was in a nearby vehicle and assisted in CPR, the newspaper said.
"I lifted him up in the air and moved him up and down," Bastidas said. "He started breathing and crying."
The baby stopped breathing again, but another round of CPR was successful and Sebastian was transported to Jackson Memorial Hospital's Pediatrics unit where he was listed in stable condition.