Lysimachia filifolia is a rare species of flowering plant in the Primulaceae known by the common name Wailua River yellow loosestrife.^[1] 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.^[2] The plant grows on the faces of waterfalls in wet forest habitat.^[2][3]

There are about 130 individuals remaining on the island of Oahu, growing at waterfalls in the Koolau Range.^[3] There is a single population on Kauai, just discovered in 2008.^[3] Another population of plants on Kauai proved to be members of a different species, Lysimachia pendens.^[3]

This plant is threatened by the degradation of its habitat by feral pigs and non-native plant species such as mistflower (Ageratina riparia), ti (Cordyline fruticosa), and umbrella tree (Heptapleurum actinophyllum).^[3]