Schiedea sarmentosa is a gynodioecious, sprawling perennial herb, woody toward its base, forming densely tangled clumps. Its cylindrical stems are 4-angled or 4-ridged, 15–50 cm long, often swollen at the nodes, multi-stemmed from the base, and many-branched. The stems, weakly ascending to sprawling, are green or tinged purple, glabrous below, and densely puberulent above and on the inflorescence, with internodes measuring (1.4–)2.5–5(–5.7) cm long. The simple, opposite leaves are linear or linear-elliptic, falcate, 3–5(–7) cm long, and 0.15–0.5 cm wide, with an attenuate and often slightly curved apex, gradually tapering at the base, where they frequently connate around the stem. Leaf blades are typically spreading or reflexed, thin, membranous, and slightly revolute, especially toward the base. They feature one or three principal veins, with the midvein excentric, and surfaces sparsely to moderately puberulent, especially along the adaxial vein and near margins, bearing white-opaque, occasionally gland-tipped hairs. Petioles are weakly to conspicuously connate; stipules are absent. The inflorescences are terminal, laterally contracted, 8–12 cm long, bearing 13–35 ascending to spreading flowers on short lateral branches. The inflorescence and pedicels are moderately puberulent. Bracts along the central axis are green, linear to subulate, and 5–35 mm long, while those of branches and flowers are shorter (1–1.5 mm). Pedicels measure 3–6 mm long and are slender and slightly flattened. Flowers can be bisexual or pistillate. Hermaphroditic flowers possess narrowly ovate, green or purple-tinged sepals 2.8–3.5 mm long, reflexed at the base, deeply navicular distally, moderately puberulent with gland-tipped hairs, and scarious, weakly lacerate margins. Pistillate flower sepals are similar but slightly shorter (2.1–2.8 mm). Petals are absent. Hermaphroditic flowers have ten stamens with subequal filaments (3.4–4.1 mm) and yellow anthers (0.75–0.9 mm). Pistillate flowers bear vestigial stamens with short filaments (0.4–0.9 mm) and nonfunctional anthers. The superior ovary is single-celled, with free-central placentation, and typically has three (rarely four) styles. The fruits are narrowly ovoid capsules, 2.3–2.7 mm long, containing orbicular-reniform, compressed seeds ~0.3 mm long, with transversely rugose surfaces. Seeds have a starchy perisperm but lack a true endosperm. It is found on steep rocky cliffs and ledges in dry forests and shrublands between 500-1000 meters.