Post oak is a small to medium-sized tree with a broad, rounded crown and stout, sometimes contorted branches.
Leaves are alternate, simple, 4–7 inches long, 3–4 inches wide, leathery; with 3–5 lobes, middle lobes squarish, resembling a cross, the end lobe often 3-notched, notches between lobes deep, rounded; upper surface dark green; lower surface paler, with tiny star-shaped hairs.
Bark is gray, irregularly grooved, ridges narrow, rough with platelike scales.
Twigs are stout, densely hairy during most of the season.
Flowers April–May. Male and female flowers are on the same tree; male flowers in drooping catkins, female flowers small and in leaf axils.
Fruits September–October, acorns solitary or paired; nut brown, broadest at the base and tapering to a rounded tip ½–¾ inch long, less than ½ inch wide; cup covering ⅓–½ of the nut, bowl-shaped, hairy on the outside; scales thick, flattened, or somewhat indented, hairy; acorns ripen in autumn of the first year.