Blackjack oak is a small to medium-sized tree with a rounded, irregular crown; distinctive bark; and a tendency to retain dead branches on the middle to lower part of the trunk. It is well limbed along the entire length of the trunk.
Leaves are distinctively wedge- or bell-shaped; alternate, simple, bristle-tipped, leathery, and shallowly 3-lobed. Upper surface is dark green and shiny; lower surface is yellow-brown or yellow-green, with tan to brown hairs. Several leaves persist in winter.
Bark resembles alligator hide; blackish, very rough, with square or rectangular blocks.
Twigs are stout, stiff, grayish-brown, and densely hairy at first, smooth later; buds reddish-brown, narrowly cone-shaped, hairy.
Flowers April–May; in catkins.
Fruits September–October, a yellow-brown acorn, 1/2 to 1 inch long; cup deep, turban-shaped, red-brown, scales loose, hairy, covering 1/3 or 1/2 of the nut. Stalk very short. Acorns ripen in autumn of second year.