Lysimachia filifolia is a rare species of flowering plant in the Primulaceae known by the common name Wailua River yellow loosestrife. It is endemic to Hawaii, where there are two populations on Oahu and one on Kauai. It is a federally listed endangered species of the United States. This is a small shrub growing up to about half a meter in maximum height. The leaves are very narrow to threadlike and the flowers are reddish purple. The plant grows on the faces of waterfalls in wet forest habitat.