Begonia pseudodendron is most similar to B. cuatrecasasiana L.B.Sm. & B.G.Schub. but differs in its white to pink, glabrous tepals (versus orange tepals that are sparsely pilose on the outside), the rounded apices of its staminate tepals (versus long acuminate apices), and its pistillate flowers with two tepals (versus three) and that lack bracteoles.

Caulescent terrestrial herb, lacking a tuber or rhizome. Stem erect or scrambling, to 10 m tall, woody at the base, frequently branching, internodes 2–10 cm, 4–6 mm in diameter, glabrous, smooth, green-flushed red, browning with age. Stipules early deciduous, lanceolate, 2.5–10 × 0.5–4 mm, apex acute, margin entire, aciliate. Leaves alternate, basifixed, straight; petioles joining blade in the direction of the main vein, 2–11 mm long, glabrous; blade subsymmetrical, oblanceolate to elliptic, 7.5–16.5 × 3–6.2 cm, apex short-attenuate, base decurrent, rounded on the broad side of the lamina, cuneate on the narrow side of the lamina, margin entire, serrate towards the apex, ciliate, upper surface green, glabrous, lower surface glabrous, green, pinnately veined, 7- to 9-veined on the broad side of the lamina, 5- to 7-veined on the narrow side of the lamina. Inflorescences: many, axillary but often appearing terminal, arising close to the apex of the stem, erect, a 6–8 times dichotomous cyme, protandrous; peduncle 1.5–4.5 cm long, internodes elongate, > 10 mm long, glabrous, pale green–flushed red; pedicels of staminate flowers 5–15 mm long, glabrous, white to pale green; pedicel of pistillate flowers 9–18 mm long, glabrous, white to pale green; bracts early deciduous, unknown. Staminate flowers: tepals 4, white to pink, outer two ovate, 3–6 × 2–4 mm, apex rounded, margin entire, glabrous; inner two narrowly oblanceolate, c.4 × 1.5–2 mm, apex rounded, margin entire, glabrous; stamens c.20, free, filaments 0.2–0.4 mm long, anthers symmetrically basifixed, oblong, 1–1.5 mm long, dehiscing via lateral slits, connectives extended to 0.2 mm. Pistillate flowers: bracteoles lacking; tepals deciduous in fruit, 2, projecting, white or pink, ovate to lanceolate, 4–12 × 3–10 mm, apex rounded, margin entire, glabrous; ovary body narrowly ovoid, 2–4 × 1–2 mm, white, glabrous, 3-winged, wings white, unequal, largest wing band-like to triangular, apex obtuse, margin entire to lacerate, 4–5 mm tall, 3–8 mm wide, smaller two wings semicircular, margins entire, apex rounded, 4–5 mm tall, 0.5 mm wide; 3-locular, placentae axile bifid, bearing ovules on both surfaces; styles 3, free, 3–6 mm long, bifid from half their length and swollen at the bifurcation, stigmatic papillae in a twice-twisted spiral band. Fruiting pedicel to 24 mm long. Fruit pendant, body broadly ovate, expanding to 8 × 4 mm, light brown, glabrous, largest wing expanding to 12 × 20 mm, triangular, ascending, shorter two wings expanding to 10 × 2 mm. Seeds unknown.