The accident occurred at around 5:30 p.m. in Little Rock Township near River Road and Blackhawk Road.
The car flipped over after hitting the hayride wagon, which was carrying roughly a dozen people at the time, according to the Plano Fire Department.
The 11 passengers injured were taken to four hospitals.
Rush Copley Medical Center received three patients. They said one is in critical condition.
There were two people in the car, and they were both treated at the scene.
Police are looking into who was at fault in the accident.
An investigation into the crash is ongoing.