Investigators say the child was left on a doorstep in the city's East Liberty neighborhood Monday night.
A passer-by, Jonetta McCowan, stopped after hearing arguing and retrieved the baby. She tells WTAE-TV that she bundled the child in her clothes and took him to police.
Investigators say the boy was taken to a hospital in good condition. Police Commander RoShelle Brackney says he was in serious danger because of the cold.
Children and Youth Services agents are trying to find the child's mother.