Witnesses saw the African-American woman, believed to be in her 20s or 30s, pushing a stroller out of a northbound 1 train at Columbus Circle on Monday (July 7) morning at about 11:50am.
The woman then darted back onto the train just before the doors shut, leaving the baby - who is believed to be around seven months old - on her own.
CCTV footage released by the NYPD shows the woman entering the subway with the baby in the stroller earlier in the day.
Story continues after the video.
A 33-year-old bystander stayed with the baby for 20 minutes, waiting for the woman to return.
When no one came to claim the child, the witness contacted police who took the baby to Transit District 1 Police Station.
When she was found, the child had a rash on her face and bandages on her hand but is said to be in good health.
Neither the woman or the child has yet been identified.
The baby was wearing a red and white blouse with red leggings and pampers.
It's unclear if the mother left her child on the platform on purpose or if something caused her to run away.