A toddler was left behind today at an Upper East Side park after she was separated from her day camp group, police said.

Good Samaritans saw the 3-year-old wandering alone in St. Catherine’s Park on First Avenue and East 67th Street just after 2 p.m. and notified cops, police said.

The little girl was separated from the group of 28 children who had come to the play at the park from a nearby school, cops said.

Police say the child was only missing for 20 minutes when someone in the park called them — the nearby school did not notify cops.

Counselors from the school were called and they picked up the child, who was reunited with her parents at the school, cops said.