Police say two store security guards noticed the little boy in the car at about 1 p.m. Sunday outside the Whole Foods Market at the Charles River Plaza.
Outside temperatures were in the 80s at the time.
The alert baby was taken to Massachusetts General Hospital as a precaution.
When they found the mother, she said she forgot the child was in the back of her car.
She was not charged but police say they will refer the incident to state child welfare authorities.
(Copyright 2012 by The Associated Press. All Rights Reserved.)