The cats were lobbed out in full view of other drivers and onlookers in the town of Ripon in what police called an “extremely distressing incident”.
Although both animals were rushed to the vets, they later died from catastrophic injuries. They are understood to have been about six weeks old.
The wife of a van driver who saw cats being thrown out of the car and attempted to save them said he had been left “devastated” by the “mindless act of cruelty”.
Susan Lund told the website Yorkshire Live: “Why would they do that? If it's just a case of getting rid of unwanted kittens, there are a billion other ways of doing it. This is just on a different level…what in God’s name possessed [them]?
“I pray that there aren’t anymore laying in agony at a roadside.”
North Yorkshire Police said it was appealing for witnesses to the incident on Thursday at about 2pm at the town’s Bondgate Green.
A spokesperson said the vehicle in question was brown or gold but gave no details of make or model.
Anyone with information is asked to call 101 quoting reference number 12200204632.