Lysimachia hillebrandii is a sprawling shrub with 40–250 cm long stems. These stems feature reddish-brown, striate bark and are branched, often with numerous short lateral branches. Young stems are rusty tomentose but become glabrate as they mature. The leaves are simple and alternate, varying from well-spaced to closely spaced and sometimes appearing somewhat imbricate. The leaves may be arranged in ternate clusters on young or lateral branches. The leaf blades are broadly elliptic-ovate to narrowly or broadly elliptic or elliptic-lanceolate, measuring (22–)25–90 mm long and (8–)10–45 mm wide, with an abruptly acuminate to attenuate apex and a broadly cuneate to attenuate base. Initially sparsely pilose, the leaf surfaces soon become glabrate; the blades are thin yet coriaceous. The margins are entire, and the veins are usually conspicuous and slightly raised on the lower surface. The petioles, measuring 4–10 mm long, are ± brown pilose initially but become glabrate over time. Stipules are absent. The flowers are solitary, located in the axils of sometimes reduced leaves. They are bisexual (perfect) and actinomorphic, with a floral formula of 5–8-merous. The pedicels are 12–37 mm long, elongating to 20–45 mm in fruit, and are ± rusty pilose. The calyx comprises 5 sepals, with calyx lobes ranging from broadly ovate to lanceolate and measuring 4–8(–9) mm long. The corolla is reddish-purple to burgundy, with darker veins, and is campanulate, measuring 10–20 mm in length. The lobes of the corolla are obovate to narrowly obovate and imbricate. The stamens are as numerous as and opposite the corolla lobes, with filaments connate approximately ⅓–½ their length and adnate to the corolla tube. The anthers are dithecal. The ovary is superior, 1-celled, with free-central placentation and a single style. The fruit is a broadly ovoid to subglobose capsule, 8–9 mm long, and regularly dehiscent. The seeds are numerous, nearly black, irregularly shaped, 1.4–2.0 mm long, with slightly winged margins and a crustaceous seed coat. It grows in open sites within mesic to wet forests between 610-1,500 meters.