Police in Bucks County say a Bensalem officer saw the 18-month-old boy alone in the car outside the Parx casino at about 9 p.m. Saturday.
Officers said the temperature was still 88 degrees with 62 percent humidity, and it was hotter inside the car than outside.
Police said surveillance video indicated that the driver had been inside for almost 40 minutes.
Authorities said 44-year-old Danny Ngo of Philadelphia was arrested on misdemeanor charges of recklessly endangering another person and endangering the welfare of a child.
A listed number for him could not be found Tuesday night; it was unclear whether he had an attorney.