The incident occurred this afternoon on Dumbarton Road, close to Boquhanran Road, in West Dunbartonshire.
She was taken to the Royal Hospital for Sick Children in Glasgow and is fighting for her life after suffering serious injuries.
Police are seeking neighbours or passersby who saw her at the window in the moments before she fell.
Hospital staff have described the girl’s condition as ‘critical’.
Detective Inspector Steve Martin said: “The little girl has sustained very serious injuries and we are trying to establish the full circumstances surrounding the incident.
“As such we are keen to speak to anyone who witnessed the child fall or anyone who has seen her at the window prior to her falling.
“A number of people were in the area at the time and came to the assistance of the girl and it is important that we speak to all of these people, therefore if any of these people left before speaking to police, we urge them to contact police immediately.
“I also ask any motorists who were in the vicinity around 2:10pm today to check their dashcam footage to see if they have captured the incident.”
The Scottish Ambulance Service sent an ambulance, a trauma team, and its special operations response unit to the scene.
A police spokeswoman said: “At 2:10pm on Wednesday 10 April, police were called to a report of a two-year-old girl having fallen from a window on Dumbarton Road, near to Boquhanran Road, in Clydebank.
“Emergency services attended and she has been taken to the Royal Hospital for Children in Glasgow where her condition is unknown.
“Inquiries are ongoing.”