Gunnera petaloidea is a giant herb that grows to heights of 1–6 meters. The stems are decumbent, measuring 2–6 meters long and approximately 12–15 cm in diameter. The leaves are simple and alternate, with cordate-reniform blades about 40–100 cm long and 20–90 cm wide. The leaf bases are not peltate, with a V- to U-shaped basal sinus. The upper surfaces of the leaves are glabrous to very sparsely hirsute, while the lower surfaces are brownish tomentose when young and sparsely to densely hirsute on the raised reticulate veins. The upper surfaces are dark green, and the lower surfaces are pale green. The leaf margins are 7–11-lobed and usually crenate-dentate, sometimes crenate or denticulate, and the leaves are palmately veined. The petioles are 20–100 cm long, pustulate, and usually densely hirsute at the base, becoming more sparsely hirsute toward the blade. Stipules are absent, but there are large axillary scales often called stipules, which are elliptic to lanceolate or oblanceolate, 1.5–10 cm long, and scurfy hirsute, becoming glabrate. The flowers are arranged in numerous spikes along an elongated central axis that is 2–10.5 dm long, with each spike subtended by an oblong, oblong-spatulate, or rarely narrowly lanceolate bract that is 8–35 mm long, with lacerate and usually ciliate margins. The flowers can be bisexual (perfect) or unisexual, with staminate and pistillate flowers intermingled with bisexual ones throughout the inflorescence. The calyx consists of 2 (or rarely 3) very small, sometimes rudimentary sepals that are equal or unequal, valvate, often cuspidate, ± caducous, deltate to ovate or ovate-elliptic, sometimes broadly lanceolate, 0.4–1.3(–2) mm long, with lacerate and sometimes ciliate margins and lacerate apex. The corolla has 0-2 greenish to occasionally rose magenta petals that are obovate, strongly concave, 0.8–2.2 mm long, glabrous to pilose, with ciliate to lacerate margins, usually caducous, and always absent in pistillate flowers. There are 1–2 stamens with very short filaments and dithecal anthers that open by longitudinal slits. The ovary is inferior, urceolate to depressed-ovoid, 2-carpellate, and 1-celled, with a single pendulous anatropous ovule. The styles are 2, terminal, subulate to filiform, and the stigmas are papillate. The fruit is drupaceous or nut-like, yellow to orange, subglobose, and 3-angled, measuring 2.5–4 mm in diameter. The seeds are 1.5–1.7 mm long, somewhat compressed, 4-angled, with one angle obscure, one inconspicuous, and the other two prominent. The embryo is embedded in a copious oily endosperm. It is found on wet, steep slopes, primarily in cloud-swept wet forests, between 920-1,300 meters, occasionally found as low as 700 meters or as high as 1,830 meters.