The incident happened Thursday morning at the Walmart at Kings Canyon and Peach Avenue.
Investigators say the mother came to the store to return diapers. She was turned away because she didn't have a receipt.
According to police, she said she needed to run to her car. That's when she left her baby with the security guard and never came back.
Walmart released this statement: "The store immediately notified the Fresno Police Department and have been providing all relevant information to them for their investigation."