The incident happened at about 3:30 p.m. Friday, the Chicago Sun-Times reported.
The girl, whose name was not reported, sustained traumatic injuries and was taken to a hospital, where she was listed in critical condition Saturday, the fire department said.
The girl's grandmother told police that she and her granddaughter were on the second floor of the store when she heard the girl call out to her, the Chicago Tribune reported.
The grandmother said she turned and saw the girl fall over the railing, but was unsure of caused her granddaughter to call out or fall, a police report said.
Sears spokesman Howard Riefs said in a statement Saturday that customer safety is a top priority for the store.
"Our thoughts and prayers are with the young girl and her family and we are fully cooperating with the authorities in investigating the matter," Riefs said.