A spokesperson for the force said “there were no records of any injuries” and the child is now safe.
A second toddler was also found on the road by concerned members of the public, according to Plymouth Live.
The pair had escaped from the nearby nursery run by Busy Bees, which owns 378 nurseries for children up to the age of five across the UK and Ireland.
A spokesperson for Busy Bees told Plymouth Live: “On Monday morning two children were found to have left our Plymouth centre unsupervised.
“Whilst we are relieved that both children returned safely to the centre, this situation clearly should not have happened and we are taking the fact it did extremely seriously.
“We are devastated that this was able to happen and a thorough investigation is now under way.
“We contacted the parents of the children involved, as well as all other parents at the nursery to explain the situation and we are working with the authorities and regulators to establish the facts and we will take appropriate action where required.”