It happened on Blake Boulevard, near Montreal Road and McArthur Avenue, at about 8:30 a.m. ET, said Ottawa police Const. Charles Benoit. It was about –8 C at the time.
The girl was found walking outside in the street on her own by a neighbour, who took the girl inside and called police, Benoit said.
The girl was feeling cold and was treated for "very, very mild hypothermia," Ottawa paramedic spokesman J.P. Trottier said.
The girl had not suffered any injuries, but she was taken to the Children's Hospital of Eastern Ontario for a check-up as a precautionary measure, Trottier said.
Criminal charges have not been laid. The Children's Aid Society is conducting a follow-up investigation, Benoit said.
In February, a three-year-old Toronto boy died in hospital after spending hours outside when he wandered away from an apartment building wearing only a T-shirt, diaper and boots.
In that case it was about –19 C at the time, but felt more like –28 with the wind chill.