Durham Regional Police say they were called to a home in the Winchester Road East and Thickson Road area at about 4 p.m.
Police say the boy was playing on a bed, up against a window screen that gave way.
He fell nearly five metres onto concrete.
The boy was taken to a local hospital and then transported to Toronto's Hospital for Sick Children.
The child is in stable condition.
Police say no foul play is suspected.
The incident comes just days after a four-year-old girl fell from a fourth-storey window in Mississauga.