Black locust is a medium-sized tree with an irregular, open crown and some low branches, as well as several large upright branches.
Leaves are alternate, feather-compound, 8–14 inches long, with 7–19 leaflets; leaflets ½–2 inches long, with tiny bracts at the base, the margin entire; upper surface bluish- to dark green, dull, smooth; lower surface paler, smooth except with hairs on the veins.
Bark is reddish-brown to almost black, with deep grooves and long, flat-topped, interlacing ridges.
Twigs are dark brown, stout, zigzag, brittle, smooth, angled with a pair of small spines where the leaves attach.
Flowers in May–June; pea-shaped, showy, white, fragrant, in loose, drooping clusters 4–5 inches long; each flower has five petals, with a yellow blotch on the inside of the uppermost petal.
Fruit in flattened pods 3–5 inches long and about ½ inch wide, smooth, reddish-brown, splitting along both sides; seeds 4–8, spotted, flattened; pods often persist on the tree through winter.