Paper mulberry, a common East Asian tree used for paper making, is propagated across the Pacific for making barkcloth, a practical and symbolic component of Austronesian material culture. Using chloroplast DNA sequences, we demonstrate a tight genealogical link between its populations in South China and North Taiwan, and South Taiwan and Remote Oceania by way of Sulawesi and New Guinea, presenting the first study, to our knowledge, of a commensal plant species transported to Polynesia whose phylogeographic structure concurs with expectations of the "out of Taiwan" hypothesis of Austronesian expansion. As a commensal plant likely transported across the full range of Austronesian expansion from South China to East Polynesia, paper mulberry may also be the most widely transported fiber crop in human prehistory.