A kitten has lost part of its leg after it was thrown from a moving van in Perth's north and the public has been asked to help track down the culprits.

A small white van was travelling south on Marmion Avenue in Quinns Rocks just after 3:00pm (AWST) on Monday.

Just before the vehicle went through a roundabout at Hester Avenue, the passenger of the vehicle threw the black and white kitten out of the window.

Part of the cat's rear leg was ripped off and the rest of the limb may need to be amputated.

It also suffered injuries to its face and front legs.

A woman driving behind the van stopped to pick it up.

The RSPCA wants to speak to anyone else who may have witnessed the incident and who may have a description of the van or its number plate.

"This type of behaviour is outrageous, and we're seeking assistance from the public to provide information regarding this incident," RSPCA WA Deputy Chief Inspector Sharon Morgan said.

The RSPCA can be contacted on 1300 278 3589.