The incident took place in Giaveno, a town 30km from Turin, during the local carnival parade.
One of the floats, which are typically covered in lavish displays, often with a political theme, ran over the woman on Sunday afternoon.
It is not clear whether the 19-year-old was in the street watching the carnival or fell off one of the floats. Witness reports varied, with some passersby saying she had been sitting on one of the music speakers on the float shortly before the accident.
Some of the spectators at the event gave the woman medical assistance at the scene before she was taken to a Turin hospital by helicopter, suffering from haemorrhage, trauma to the spine and suspected bone marrow injuries.
The location of Giaveno is shown on a map.
An initial operation was carried out successfully, police said later on Sunday as reported by TorinoToday, but the teenager's condition is still very serious.
Giaveno's carnival is one of the last to take place in Italy, with most towns already having finished their celebrations.
The parade took place from 2pm to 3:30, with bands, masked marchers, and the allegorical floats. Most of the floats were burned in the town square later in the afternoon, as is carnival tradition.
The float involved in the accident was one of the last in the procession, and was decorated on the theme of Italy's banking crisis. It has been confiscated by police, who are working to establish how the incident occurred.
Police have questioned the driver of the float, who was also tested for alcohol. The tests came back negative.