The toddler will be turned over to a relative, Daily Tribune Media reported.
A care worker spotted the boy wandering around the Thousand Oaks Trailer Park in Wisconsin Rapids on Friday morning. The child was wearing a short-sleeved shirt, diaper and one sock, Police Lt. Brian Krzykowski said.
Officers went to the boy's home and woke the father, who didn't know his son was missing. Wood County Human Services took custody of the boy and will turn him over to a responsible relative, Krzykowski said.
It was the second case in a week in which a Wood County child was left unsupervised.
A Marshfield man told police he found a 2-year-old girl walking outside alone on Wednesday about 2:15 a.m. She was barefoot and in pajamas.
Police found that the father, a 35-year-old Marshfield man, had left his daughter unsupervised for about an hour. The girl was turned over to her grandparents, and police have asked prosecutors to charge the man with child neglect.