The female tortoiseshell cat, named Pineapple, was rushed to the vets after being found by a shocked passerby.
She was found to be lactating - suggesting she may have recently given birth to kittens.
A spokesman for the Scottish SPCA said: "We were contacted on August 26.
"An onlooker had brought the cat to them the previous evening, having seen her being thrown from the vehicle between 9pm and 9.30pm."
Scottish SPCA inspector Robyn Gray said: "We were contacted by the vet first thing in the morning.
"They informed us that a member of the public took the female tortoiseshell to the practice after witnessing her being thrown from the car near The Pineapple building in Airth.
"Upon examination, she was found to be lactating which could indicate she has recently given birth.
"We are concerned for the kittens welfare at this stage.
"The cat, who has been affectionately named Pineapple, is currently recovering at one of our animal rescue and rehoming centres.
"We are keen to speak to anyone who can help us establish what happened to Pineapple."