Lysimachia filifolia is a rare species of flowering plant in the Primulaceae known by the common name Wailua River yellow loosestrife.

[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] 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).