The child was shot in the lower neck, police said, and is in critical condition at Stroger Hospital.
"This is unacceptable", police Sgt. Rocco Alioto said.
It was not immediately known if the wounded man was with the girl or was the one targeted by the shooters.
The teen was amassing sweet with a 31-year-old man when a bunch of males opened hearth on the pair within the metropolis's Little Village neighborhood at about 5:30 p.m., based on The Chicago Tribune.
Authorities said the man and girl were walking along a street when a group of men approached and one of them shot at the pair.
A 7-year-old girl out trick-or-treating in a bumblebee costume was critically injured after being struck by apparent stray gunfire on Chicago's West Side, police said.
"Because a group of guys wanted to shoot at another male while there's a street full of kids trick-or-treating". "The girl's father was screaming, 'My little girl's been shot'".
"We heard the shots. four shots, and I went outside", said Lali Lara, who works in a cellphone store. Lara told the newspaper she held the girl's hand and pressed against her chest to stop the bleeding. The intended target was a member of the Latin Kings gang, police said, according to a report in the Chicago Tribune. "She was holding my hand for three minutes and then she let me go".
Police are investigating and there have been no instant arrests.
"Helpful community info is already coming in but we need more", Guglielmi said on Twitter.
The police spokesman noted that the injured girl was taken to the hospital and is fighting for life.
Trick-or-treaters dressed in Halloween costumes walked past the crime scene Thursday night after police taped off parts of the street.