Lysimachia filifolia is a small shrub, typically growing 15–50 cm tall. The young stems are either viscid, glabrous, or sparsely pubescent. The leaves are simple and alternate, although they are often so closely spaced that they may appear sub-opposite or sub-verticillate. The leaf blades are linear–filiform, measuring 15–54 mm in length and 0.3–1.8 mm in width, with a bluntly acute apex. The surfaces are either glabrous or sparsely puberulent, and the margins are smooth. The leaves are sessile and possess a single vein with no stipules present. The flowers are bisexual (perfect) and actinomorphic (radially symmetric), typically solitary within the leaf axils. They are 5–6-merous, with about 18–30 mm long pedicels, which elongate to 45–60 mm in fruit. The pedicels can be either viscid or sparsely puberulent and are bracteate, though bracteoles are absent. The calyx comprises five sepals, with narrowly lanceolate lobes 4–5.3 mm long, enlarging to approximately 6 mm in fruit. The corolla is reddish-purple and campanulate, measuring 6–10 mm in length, with narrowly obovate lobes that are imbricate. The stamens are as numerous as the corolla lobes and are positioned opposite them. The filaments are connate at the base and adnate to the corolla tube, while the anthers are dithecal. The fruit is an ovoid capsule, thick and hard, measuring 5–6 mm in length and regularly dehiscent. The seeds are numerous, nearly black, irregularly shaped, about 0.7–1.2 mm long, and have a crustaceous seed coat. It grows on mossy banks at the base of cliff faces within the spray zone of waterfalls or along streams in lowland wet forests between 240-680 m.