Scarlet oak is a medium-sized tree with a long, straight trunk, an open, narrow crown, and sometimes persistent dead branches on the lower trunk.
Leaves are alternate, simple, 3–7 inches long, with 7–9 lobes extending more than halfway to the central vein, the notches rounded and C-shaped, the lobe tips with large, bristle-tipped teeth. The upper surface is bright green, shiny, and smooth; the lower surface is paler, sometimes with tufts of rusty hairs at the axis of main veins. Leaves turn scarlet in autumn.
Bark has shallow grooves and irregular ridges, becomes scaly with age.
Twigs are slender, greenish at first; orange-red or brown with age, smooth or hairy. Bud scales have whitish hairs near the tip.
Flowers April–May, in catkins.
Fruits September–October, acorns solitary or paired; nut brown, broadest near the base, ½–1 inch long, top sometimes with concentric rings; cup covering about ½ of the nut, cup scales thick, flattened, sometimes warty, shiny, smooth. Acorns ripen in autumn of the second year.