Cow vetch is in the family Fabaceae and similar to a pea in growth habit with climbing stems growing to 150 cm long, perennial, sending out noose-like branched tendrils from the tips of its leaves when it contacts another plant and securely fastens itself. This can cause "strangling" of smaller plants. An individual plant may reach a length (or height) of 2 m with a white taproot, which may extend up to 1 m. The leaves are 3–8 cm long, pinnate, with 8–12 pairs of leaflets, each leaflet 5–10 mm long.
The plant is fast-growing and flowers prolifically, sending out 10 to 40 flowered one-sided racemes cascading pea-flower shaped purple to violet flowers from the leaf axil during its late spring to late summer flowering period. The flowers are mostly visited by bumblebees; when the flowers drop off and tiny bright green seed pods 10–20 mm long, start to form. Cow vetch is very similar to hairy vetch (Vicia villosa), but is distinguished from the latter by its smooth stem.
The seed pods are 2 cm long and contain 6 to 8 seeds. They resemble those of a very small pea. The tiny seeds within are ripe when the pods have turned black. Unripened seeds are swollen and have a green tint to them, but they unswell when they become ripe. The seed pods vary from light brown to dark brown with black spots.