The incident happened Friday evening in Chicago's Magnificent Mile shopping area, the Chicago Tribune reported.
Police said a 51-year-old woman from Muskegon, Mich., who was with two other women, was confronted by a group of teenagers who held her and took her iPhone from her pocket.
The Florida girl said she and her mother witnessed the incident and that she decided to step in.
"They felt endangered, but I'm a person who tries to help and I wanted to help the lady," she said. "My mom tried to stop me three times."
The Florida teen said she screamed "Give her back her phone," at the suspects, who denied having the phone.
"That's when the girl charged at me," said the Florida teen, adding that one of the suspects punched her in her head, causing a bruise and some swelling.
"I wasn't really thinking about what might happen; I just wanted to help her," the teen said.
Police said they were called to the scene and arrested eight suspects, who were charged with robbery. The youth suspected of punching the Florida girl was also charged with battery.