The new species is similar to Begonia rugosula Aver. in the small leaves and climbing habit, but different from it in the round leaf top (vs. acute leaf top), flat surface on the adaxial surface of the leaves (vs. uneven surface in B. rugosula), ovate interior petals (vs. narrowly obovate or narrowly obovate), 2-branching stigma (vs. slightly spherical).

VIETNAM. Bac Kan Province, Nari county, Kim Hy National reserve, 22°16'22"N, 106°03'23"E, 504 m a.s.l., 18 April 2016, in flowers, Y.M. Shui, W.H. Chen, C. Liu, H.Q. Nguyen, H.T. Nguyen, N.Q. Chuong CK1210 (holotype, KUN!; isotype, CPC!).

Rhizome: slender, 2.5–7 mm long, 1.5–2 mm in diam. Stipule reddish, triangular, subglabrous, 1–1.8 × 8–1.2 mm wide. Leaves: petiole reddish, erect, villous, 1.5–3.5 cm long, 0.5–1 mm in diam.; blade asymmetric, widely ovate or rotund, 2.2–2.6 × 1.6–1.8 cm, base cordate, apex obtuse to acute, margin slightly serrulate, ciliate; adaxially dark green with white line or stripes along veins, strigose; abaxially slightly greenish striated and short strigose along primary and secondary veins; venation palmate, 4–5 primary veins, 2 or 3 secondary veins brunching dichotomous, tertiary veins reticulate. Inflorescence: dichasial cyme; peduncle 2–3.5 cm long, glabrous; bracts caducous, greenish, triangular, glabrous, apex acute, margin entire. Staminate flower: pedicel ca. 1 cm long; tepals 4, widely ovate, white inside, pinkish outside, both sides glabrous; outer tepals 2, broadly ovate, glabrous, base cuneate, apex obtuse or rounded, margin entire; inner tepals 2, ovate, base cuneate, apex acute to obtuse, margin entire; androecium actinomorphic, stamens numerous, filaments longer than anthers, fused at base; anthers yellow, obovoid, apex emarginate, with longitudinal slits. Pistillate flower: pedicel ca. 1 cm long; tepals 3, white, both sides glabrous; outer tepals 2, broadly ovate, ca. 5 × 6 mm, abaxially slightly pinkish with red spots, base rounded, apex obtuse or rounded, margin entire; inner tepal 1, elliptic to oblong, ca. 6 × 4 mm, base cuneate to rounded, apex obtuse, margin entire; styles 3, free, stigmas bifid with twisted bands; ovary slightly pinkish with red minute spots, ca. 5 mm long, ca. 3 mm in diam. (wings excluded), glabrous, 1-loculed; placentation parietal, 2-branched each placenta; Capsule nodding, trigonous-ellipsoid, subequally 3-winged; abaxial wing lunate, ca. 5 ×3 mm, slightly larger than lateral wings; lateral wings narrowly lunate, ca. 5 × 2 mm. Seeds ca. 0.15 mm long, oblong.

Flowering in April–May, fruiting in May–June.

The epithet refers to the small size of leaves.

The species just grows in limestone crevices of cliffs.

The species occurs exclusively in Nari county of Bac Kan Province in Vietnam.

In Begonia sect. Coelocentrum, the new species is one of the smallest species in morphology. Its closest species with limited morphology is Begonia regosula Aver. also from Bac Kan Province (Averyanov and Nguyen 2012). In the new species, however, the wider interior petals and the obvious bifid stigma can separate it from B. regosula (Fig. 6).