Lobelia gaudichaudii typically reaches heights of 1–3 meters. The stem is 0.8–2 meters tall and 1.3–5 cm in diameter, featuring distinct transversely rhombic cicatrices (leaf scars) that are 2.5–45 mm tall by 7–11 mm wide with acute corners, giving it a unique appearance. The leaves are simple and alternate, typically oblanceolate in shape, measuring 9.5–19.5 cm long and 1.1–2.8 cm wide. They have an acute or acuminate apex and an attenuate base and are densely pubescent on the lower surface along the midrib, though occasionally they can be glabrous. The margins are distinctly revolute, and the plant does not have stipules. The flowers occur in a tall, unbranched inflorescence measuring 0.4–0.8 m in height and cylindrical in outline. This inflorescence is green and glabrous, with a peduncle sparsely covered with decreasingly sized sterile bracts. Each flower has a pedicel 20–40 mm long and is accompanied by floral bracts that are 18–32 mm long and 6–8 mm wide, ovate or lanceolate in shape. The hypanthium is broadly obconic or campanulate, measuring 6–10 mm long by 7.5–12 mm in diameter. The calyx is 5-lobed with lobes 13–18 mm long and 4.5–7 mm wide, elongating to form a significant part of the flower’s structure. The corolla is red and zygomorphic, dorsally split nearly to the base, 60-56 mm long, with a tube 20-30 mm long and 6-11 mm in diameter, expanding towards the mouth. The upper lip is 2-lobed, while the lower lip has three lobes. The fruit is a capsule measuring 23–30 mm long by 11–19 mm in diameter. It dehisces at the apex through two valves. The seeds are numerous, chestnut-brown, 1–1.2 mm long, and 0.7–0.9 mm wide, embedded in oily endosperm. It is known only from montane wet shrublands and thrives at an elevation of 805-960 meters.