Two Good Samaritans allegedly found the baby, between 1 and 2 years old, near the intersection of Western Avenue and Washington Boulevard around 2:30 a.m.
Police say the mother of the baby returned to the location as officers arrived. That's when she was arrested for child abandonment.
The baby was taken by LAFD paramedics to a local area hospital to make sure she had not been harmed by sitting out in the cold.
It remains unknown how the long the baby was left unattended on the sidewalk or where the mother was. Officials say the temperature in the area at the time of the incident was 52 degrees Fahrenheit.
CNS contributed to this report.