A partial postcranial skeleton (vertebrae and ribs) of an indeterminate sirenian is described from Selminum Tem cave in the Hindenburg Range, Western Province of Papua New Guinea. It was derived from a section of the Darai Limestone dating to the Burdigalian-Serravallian (early-middle Miocene) and representing shallow platform carbonates. The thoracic vertebrae are remarkably small, being comparable in size to the vertebrae of Nanosiren garciae and implying small body size, although it is uncertain whether the specimen represents a diminutive adult or juvenile individual. These fossils represent the geologically earliest mammal recorded from the island of New Guinea and the earliest evidence of Sirenia in Australasia. Thus, this fossil evidence provides a minimum date (∼11.8 Ma) for the earliest presence of sirenians in Australasian coastal marine ecosystems, as well as their primary food source, seagrasses.