Round-leaved catbrier is a climbing, perennial woody vine to 20 feet long with tendrils and stout spines, sometimes forming tangled thickets.
Leaves alternate, simple, 2–6 inches long, 1 to 6 inches wide, from egg-shaped to circular or heart-shaped, sometimes lance-shaped; margins entire or occasionally with scattered teeth; upper surface dark green, smooth; lower surface smooth, not paler than the upper surface. Blade is thin and leathery. Leaf stalks are shorter than the leaf blades and longer than the stalks of the flower and fruit clusters.
Stems are bright green to brown, smooth or nearly so, usually 4-angled; prickles are green or with a dark tip, flattened, and with a broad base, 1/8 to 1/2 inch long, scattered, and stout; tendrils arising in pairs at the base of leaf stalks.
Flowering is in late April–May. Flowers are greenish yellow, small, with male and female flower clusters on the same plant, in clusters; petals 6. The flower cluster’s stalk is shorter than or equal to the length of the leaf stalk, ¼–¾ inch long.
Fruits mature September–October. Fruits are round, bluish-black berries, covered with a grayish-blue coating, about ¼ inch thick. Occur in clusters of 3–25. The cluster stalk is shorter than or equal to the length of the leaf stalk, ½–1½ inches long. Seeds 2 or 3 per fruit.
Similar species: If flowers and fruit cannot be seen, and you are only looking at vegetative portions of the plants, this species can be hard to distinguish from S. bona-nox: both have shiny leaves with the veins thickened on the undersurface. However, the leaves of S. bona-nox tend to be thicker, semi-evergreen, and somewhat leathery, while those of S. rotundifolia are thinner and herbaceous or papery.