Pin oak is a large tree with a tall, straight trunk and pyramidal crown. Lower limbs droop, middle limbs are horizontal, and top limbs slant upward.
Leaves are alternate, simple, 4–6 inches long, broadest in the middle; lobes usually 5–9; notches rounded, deep, 2/3 or more to the central vein; each lobe with 2–4 sharp-pointed teeth, bristle-tipped. Upper surface dark green, shiny; lower surface paler, smooth, with tufts of hair in the vein axils. Deep scarlet in fall.
Bark light brown, smooth, shiny; becomes gray-brown, shallowly grooved and slightly roughened with closely flattened scales with age. The many limbs make pin knots in the lumber.
Flowering April–May, in catkins.
Fruits September–October, acorns solitary or in clusters of 2 or 3, often striped, 3/8 to 1/2 inch long, hemispherical; the shallow cup covers 1/4 to 1/3 of the nut. Fruit bitter, ripening in autumn of second year.
Similar species: Northern pin oak (Q. ellipsoidalis): acorns are longer; known only in northern Missouri.