The child spent eight hours in the car on one of the hottest days in southern Sweden before his father returned, the Swedish news agency TT reported.
"The father explained that he started thinking about whether he actually left his son at day care, and when he came back to the parking space, the boy was sitting there," chief prosecutor Mats Svensson said.
Svensson said the boy's mother usually dropped him off at day care and the father was not in the habit of doing it.
The father could be charged with aggravated manslaughter.
He was questioned initially but is reportedly too distraught for further questioning at this time, police said.