Sergeant Obakeng Mabaso said the woman was arrested on Friday night and would appear in court soon.
“She will face child abandonment charges,” she said.
The 16-month-old child narrowly escaped being hit by a truck on Thursday as he walked along the freeway.
Gauteng traffic police spokeswoman Busaphi Nxumalo said it was suspected that the child was left on the highway as there were no houses nearby.
Traffic police patrolling the N1 noticed a truck swerving and then saw the child walking on the freeway around 7pm.
The toddler was taken to a place of safety.