The five-lobed calyx of astragalus alpinus
is light green, with a sparse to moderate covering of flattened black hairs, while the petals are bicolored; variously white, pink or purple. The banner petal is light lavender-purple, white at the base and crossed by darker lines, while the two wing petals are white. The keel is purple at the tip, white below. The colored parts of the petals wither to dark blue. Flowers are produced in a loose cluster of up to 30 heads, that elongates as it matures. Fruits are pendant green pods, less than one inch long, covered by dark hairs. The lower margin of the pod is grooved.
Leaflet edges are ciliate, and the undersides sparingly hairy; the upper surfaces are glabrous. Stems generally grow sideways, staying close to the ground. The flowering stalks are leafless. This species is the most common milkvetch of the Rocky Mountains, and is also found at similar latitudes all across the globe.