Sandbar willow is a slender, upright shrub forming thickets by spreading roots, or a small tree.
Leaves are simple, alternate, very narrow, 2–6 inches long, about ¼ inch wide, thin, with scattered and unevenly spaced, gland-tipped small teeth, only 3–12 to an inch; tip pointed. Leaf stalk 1/8 inch or less, hairy. Young leaves silky-hairy beneath.
Bark is green to gray or brown, smooth; on older trunks furrowed and broken into closely flattened scales.
Twigs are slender, erect, green to brown or red, smooth or hairy, sometimes with a white, waxy coating.
Flowers May–June, flowering after leaves develop, male and female flowers in separate catkins in axils on twigs, borne on separate plants.
Fruits June–July, catkins 1½–2 inches long; capsules about ¼ inch long, oval with a beaklike point; seeds minute, attached to long white silky hairs at the base.