Cooper's milkvetch is an erect forb, 30-90cm tall, with a smooth stem, erect in stature, that usually develops a hollow, branchless area in the center. Its inflorescence is made up of several racemes that barely extend past the tallest leaves with white flowers 11-15mm long. Its fruits are sessile, erect, and spindle- or obliquely egg-shaped, 2cm x 8-18mm, inflated like a bladder, and unilocular. Its pinnately compound leaves contain 11-23, oblong to elliptic leaflets that are each 1-3cm long. The undersides are covered in many straight, stiff, and sharp hairs that are pressed flat against the leaves, stipules subtend the leaves free.
It can be distinguished from Astragalus canadensis by its free stipules, vs joined around stem as in A canadensis.