The force said inquiries are at an early stage after the incident was reported at around 9pm on Sunday.
Part of the park near Motherwell has been cordoned off while officers investigate the area.
A Police Scotland spokeswoman said: “Officers are carrying out enquiries after reports of a 16-year-old female being seriously sexually assaulted.
“Enquiries are at an early stage.”
The park is used for sporting events and contains the M&Ds theme park.