Lysimachia kalalauensis is a sprawling or scandent shrub with bark that is reddish-brown and striate and stems that are 2–3 meters long, few-branched, with young ones displaying a rusty tomentose texture that soon becomes glabrate. The leaves are simple and alternate, well-spaced, with elliptic blades measuring 48–95 mm in length and 16–34 mm in width. These blades are thin, coriaceous, and have an acuminate apex, a cuneate base, and entire margins. The lower surfaces are brown pilose when young but soon become glabrate. The primary and secondary veins are red, and the petioles are 5–10 mm long, initially brown pilose but soon glabrate. The flowers are solitary in leaf axils, perfect, actinomorphic (radially symmetric), and 7–8-merous. The pedicels are 25–30 mm long, elongating to 30–45 mm in fruit, and are rusty pilose. The calyx lobes are narrowly lanceolate, and the corolla is green with a purplish-red base and purple veins, campanulate, with oblanceolate lobes. The stamens are as many as and opposite the corolla lobes, with filaments connate for about half their length. The fruit is a capsule, broadly ovoid to subglobose, regularly dehiscent, and 9–10 mm long. The seeds are numerous, irregularly shaped, 1.7–2.3 mm long, with winged margins, a reticulate surface, and a crustaceous seed coat. It thrives in shaded sites within diverse mesic forests.