The 11-year-old told detectives she was grabbed by the suspect while walking along Hinsdale Street just before 9 a.m.
She was pulled into the back of his van and sexually assaulted, she told detectives.
Police say the young girl said he pushed her out of the van and drove away.
She described the suspect as a Hispanic man about 30-years-old and wearing khaki pants.
The attack was not immediately linked by detectives to any active pattern.