Sida fallax is a prostrate or diffuse to erect shrub that grows 0.2–1.5(–3) meters tall, with stems that are glabrate to densely velvety tomentose, often bearing stellate hairs, sometimes with additional simple and glandular hairs. The leaves are simple and alternate, with lanceolate-ovate blades to oblong, elliptic, or suborbicular, ranging from 1–12 cm long. The leaf apex can be acute, acuminate, or obtuse, while the base is obtuse to subcordate. Upper leaf surfaces are glabrous or sparsely to densely pubescent, while lower surfaces are densely pubescent, making the leaves usually discolorous with bright green upper surfaces. Leaf margins are finely to coarsely serrate or crenate, with petioles measuring 0.5–5.5 cm long and stipules that are filiform to linear and 3–10 mm long. The flowers are axillary, solitary, or in groups of 2-7 per node, either in the leaves or crowded at the branch ends. They are pedicellate or rarely subsessile, with slender pedicels that can be distinct or sometimes connate in a common peduncle, articulating at 0.5-2 cm long in flower and extending to 5(-7) cm long in fruit. The flowers are bisexual, with a green or dark purple campanulate calyx (5.5–)6–9 mm long, slightly larger in fruit, strongly ribbed, and triangular to ovate, acute to acuminate lobes. The corolla is actinomorphic (radially symmetric) to moderately zygomorphic (bilaterally symmetric), yellow to orangish yellow, sometimes dark maroon at the base, and rotate, with five broadly obovate petals that are 8–15 mm long, asymmetrically 2-lobed apically, ciliate along the claw, distinct but adnate at the base to the staminal column. The stamens are monadelphous, forming an included staminal column with 2.5–4 mm long yellow filaments, glabrous or puberulent apically, and monothecal anthers. The ovary is superior, (1-)3-celled to many-celled, with carpels borne in a single whorl or rarely in seemingly superposed whorls, axile placentation, usually with 5-10 or numerous carpels each having one ovule. The style exceeds the staminal column, unbranched and stigmatically lobed at the apex or branched. The fruit is a schizocarp, with (5)6–9 one-seeded mericarps that are pale brown to black, basically trigonous, 2.8–4 mm long, with strongly rugose–reticulate lower dorsal and lateral walls, a puberulent apex, and either muticous, short-beaked, or with awns 0.3–1(–1.8) mm long, and indehiscent except for the upper 1/3 which is dehiscent. Seeds are brown or black, trigonous–obovoid, 1.7–2.2 mm long, and puberulent around the raphe. It is found in rocky or sandy coasts, raised limestone reefs, open arid lava fields, dry forests, diverse mesic forests, and rarely in low-elevation wet forests, from sea level to 1,980 meters.