Police did not release any identifying information about the person in custody.
A neighbor said the girl was stabbed more than 10 times and she claimed a man she met at a McDonald's was trying to rape and kill her.
The teen was taken to Advocate Christ Hospital in serious condition after the incident.
The attack happened June 23 in the Morgan Park neighborhood. The girl said she met a man earlier at a McDonald's at 116th and Halsted streets around lunch time.
Police said the suspect was able to talk the girl into accompanying him to a secluded wooded area in the 108000-block of South Bishop Street where he then grabbed her from behind, pushed her to the ground and sexually assaulted her. The suspect then stabbed the girl with a fixed blade kitchen knife, police said.
The victim was able to defend herself with a tree branch and the suspect then fled, according to police.
The girl knocked on Tracy Carey's door near 108th and Bishop streets, screaming for help.
"I was trying to keep her calm so I could stay calm, and she was just in a panic. And I made her sit down because she was bleeding really bad," Carey said.
Carey said the girl told her she was a runaway. She appeared to be in shock and badly hurt but remained conscious and was able to talk to her.
She gave some information about her family, but nothing about the man who attacked her.
Reggie Dunlap, another neighbor, got concerned when he noticed blood on the girl.
"She said she was stabbed over 10 times," Dunlap said. "He tried to rape and kill her."
"It just sends a message that, you know, it's still dangerous out here on the street but she's still somebody's baby and he never should have snatched her, got with her and took her to that wooded area," community activist Andrew Holmes said at the time.