The western ringtail possum or ngwayir is a species of possum found in a small area of Southwest Australia. They are a cat-sized marsupial with a stocky build, dark greyish-brown fur, pale underparts and a long prehensile tail with a whitish tip. Ngwayir forage at night through the upper canopy of trees, feeding on young leaves, flowers and fruit, especially in groves of the weeping peppermint Agonis flexuosa. Breeding occurs mainly during the winter, the single juvenile emerging from the pouch after about three months. The population has declined by more than 95% since British settlement, due to clearing of habitat, fire and the introduction of the red fox Vulpes vulpes, and is classified as Critically Endangered. The population in most areas has catastrophically declined or become locally extinct, but strongholds remain in the urbanised areas near Busselton and Albany.

The common ringtail possum is an Australian marsupial.